1. 17(2) c7 = {89}
Innies n9 = r7c78 + r9c7 = +22(3)
-> r9c7 from (57)
-> 12(2)r9 = {57}

2. Outies n478 = r3c1 + r9c7 = +9(2)
-> 10(2)c1, 12(2)r9 from [28],[57] or [46],[75]

3. Innies n1 = r2c3 + r3c13 = +10(3)
Since r3c1 from (24) -> 6 in n1 in 23(4)

4! Innies c789 = r1459c7 + r4c8 = +29(5)
Max r4c8 = 9 -> Min r1459c7 = +20 -> Max r238c7 = +8(3) (i.e., no 6,7)
-> 6 in c7 in r14c7

Whether 6 in r4c7 or 6 in r1c7 (-> 6 in r2c1) -> 10(2)c1 cannot be [46]
-> 10(2)c1 = [28]
-> 12(2)r9 = [57]

Basically cracked at this point...

5. Remaining innies n9 = r7c78 = +15(2) = [96]
-> r6c7 = 8
Also 8(2)n9 = {35} and 15(4)n9 = {1248}

Remaining Innies n8 = r789c4 + r9c5 = +19(4) and must contain (38)
-> Hidden single 9 in n8 -> 13(2)n8 = [49]
-> 9 in n7 in r9c12

6. Innies - Outies n4 -> r6c3 = r7c1 - 3
-> 9 in n4 in 22(4)
-> HS 9 in n1/c3 -> r1c3 = 9

Innies n3 = r1c7 + r3c9 = +9(2) (i.e., no 9)
-> 9 in n3 in r2c89

Remaining innies n1 = r23c3 = +8(2) must be from [17] or [35]
-> HS 9 in n2/r3 -> 12(2)c5 = [93]
-> Remaining Innies n5 = r45c6 = +15(2) = [78]
-> r5c7 = 4

7. Since r7c8 = 6 -> r6c89 = +8(2)
-> Remaining innies n36 = r14c7 + r4c8 = +18(3)
Max r14c7 = {56} = +11 -> Min r4c8 = 7
Since (78) already on r4 -> r4c8 = 9
-> r14c7 = +9(2) = [36]
-> 8(2)n9 = [53]
-> 10(2)n3 = [217]
etc.

Note - not yet used the diagonals!

1.25. Once I found the breakthrough move (Step 4) it was quite straightforward. Was that a fish? I'm never quite sure.

Prelims

a) R23C2 = {39/48/57}, no 1,2,6
b) R3C34 = {49/58/67}, no 1,2,3
c) R34C1 = {19/28/37/46}, no 5
d) R34C5 = {39/48/57}, no 1,2,6
e) R4C23 = {18/27/36/45}, no 9
f) R5C23 = {18/27/36/45}, no 9
g) R5C67 = {39/48/57}, no 1,2,6
h) R67C7 = {89}
i) R78C5 = {17/26/35}, no 4,8,9
j) R78C6 = {49/58/67}, no 1,2,3
k) R8C78 = {17/26/35}, no 4,8,9
l) R9C67 = {39/48/57}, no 1,2,6
m) 10(3) cage at R2C7 = {127/136/145/235}, no 8,9
n) 26(4) cage at R1C8 = {2789/3689/4589/4679/5678}, no 1

1. Naked pair {89} in R67C7, locked for C7, clean-up: no 3,4 in R5C6, no 3,4 in R9C6
1a. 45 rule on N9 3 innies R7C78 + R9C7 = 22 = {589/679}, 9 locked for R7 and N9, clean-up: no 4 in R8C6, no 8,9 in R9C6
1b. R9C7 = {57} -> no 5,7 in R7C8
1c. Naked pair {57} in R9C67, locked for R9
1d. R78C6 = [49/58/85] (cannot be {67} which clashes with R78C5 and R9C6, killer ALS block), no 6,7
1e. 4 in N9 only in 15(4) cage at R7C9 = {1248/1347/2346}, no 5
1f. Min R7C8 = 6 -> max R6C89 = 8, no 8,9 in R6C89

2. 45 rule on N478 1 innie R4C1 = 1 outie R9C7 + 1, R9C7 = {57} -> R4C1 = {68}, R3C1 = {24}
2a. 45 rule on N1 3 innies R2C3 + R3C13 = 10 = {127/145/235} (cannot be {136} because R3C1 only contains 2,4), no 6,8,9, clean-up: no 4,5,7 in R3C4
2b. R2C3 + R3C13 = {127/145/235} = [127/145/325] -> R2C3 = {13}, R3C3 = {57}, clean-up: no 9 in R3C4
2c. R23C2 = {39/48} (cannot be {57} which clashes with R3C3), no 5,7
2d. Max R2C3 = 3 -> min R12C4 = 11, no 1 in R12C4

3. 45 rule on C89 3 innies R348C8 = 19 = {379/469/478/568} (cannot be {289} because 8,9 only in R4C8), no 1,2, clean-up: no 6,7 in R8C7
3a. 8,9 only in R4C8 -> R4C8 = {89}
3b. Naked pair {89} in R4C8 + R6C7, locked for N6
3c. 9 in C9 only in R123C9, locked for N3
3d. R7C78 + R9C7 (step 1a) = {89}5/[967]
3e. {469/568} of R348C8 = [496/586] (cannot be [685] which clashes with R7C78 + R9C7), no 6 in R3C8, no 5 in R8C8, clean-up: no 3 in R7C8
3f. 5 in N9 only in R89C7, locked for C7, clean-up: no 7 in R5C6
3g. R5C6 = {589}, R78C6 = [49/58/85] -> combined half cage R578C6 = {58}[49]/9{58}, 9 locked for C6
3h. 10(3) cage at R2C7 = {127/136/145/235} -> R23C7 = {12/14/16/23}, R3C8 = {357}
3i. 45 rule on N3 2 innies R1C7 + R3C9 = 9 = [18/27/36/45/63/72], no 1,4,9 in R3C9

4. 45 rule on R12 3 innies R2C267 = 11 = {128/137/146/236/245}, no 9, clean-up: no 3 in R3C2
4a. 8 of {128} must be in R2C2 -> no 8 in R2C6

5. 45 rule on R1234 3(2+1) innies R34C9 + R4C4 = 9
5a. Min R34C9 = 3 -> max R4C4 = 6
5b. Min R4C49 = 3 -> max R3C9 = 6, clean-up: no 1,2 in R1C7 (step 3i)
5c. Min R3C9 + R4C4 = 3 -> max R4C9 = 6
5d. 1 in N3 only in R23C7, locked for C7, clean-up: no 7 in R8C8
5e. Killer pair 5,6 in R7C78 + R9C7 and R8C78, locked for N9
5f. 6 in N9 only in R78C8, locked for C8
5g. R348C8 (step 3) = {379/568}, 5,7 only in R3C8 -> R3C8 = {57}
5h. Naked pair 5,7 in R3C38, locked for R3, clean-up: no 4 in R1C7 (step 3i), no 5,7 in R4C5
5i. 10(3) cage at R2C7 contains 1 = {127/145} (cannot be {136} because R3C8 only contains 5,7), no 3,6
5j. 45 rule on C89 1 innie R4C8 = 3 outies R238C7 + 1, R4C8 = {89} -> R238C7 = 7,8 = {124/125}, 2 locked for C7

6. 45 rule on C6789 3(2+1) innies R1C67 + R6C6 = 11
6a. Min R1C7 = 3 -> max R16C6 = 8, no 8 in R16C6

7. 45 rule on C12 4 outies R1458C3 = 26 = {2789/3689/4589/4679} (cannot be {5678} which clashes with R3C3), no 1, 9 locked for C3, clean-up: no 8 in R4C2, no 8 in R5C2

8. 16(4) cage at R1C5 = {1267/1348/1357/1456/2347/2356} (cannot be {1249/1258} because R1C7 only contains 3,6,7), no 9

9. 45 rule on R89 4 outies R7C2569 = 14 = {1238/1247/1256/1346/2345}
9a. 8 of {1238} must be in R7C6 -> no 8 in R7C29

10. 45 rule on N5 3 innies R4C56 + R5C6 = 18 = {189/369/378/459/468} (cannot be {279/567} because 2,6,7 only in R4C6), no 2
10a. 1,6,7 of {189/369/378/468} must be in R4C6 -> no 3,8 in R4C6
10b. R4C56 + R5C6 = {189/369/378/468} (cannot be {459} because [459/945] clash with R78C6), no 5, clean-up: no 7 in R5C7
10c. 6 of {468} must be in R4C6 -> no 4 in R4C6
10d. Killer pair 8,9 in R5C6 and R78C6, locked for C6

11. 26(5) cage at R2C6 = {13679/14579/14678/23579/23678/24569/24578/34568} (cannot be {12689/13589/23489} because 8,9 only in R4C8)
11a. R5C67 = [84/93], R78C6 = [49]/{58} -> combined cage R5C67 + R78C6 = [84][49]/[93]{58} -> 26(5) cage cannot contain both of 4,5 because 5 only in R2C6) -> 26(5) cage = {13679/14678/23579/23678} (cannot be {14579/24569/24578/34568} which contain both of 4,5)
11b. 26(5) cage = {13679/14678/23678} (cannot be {23579} = [52739] which clashes with R9C6), no 5
11c. R4C56 + R5C6 (step 10b) = {189/369/378/468}
11d. 26(5) cage = {13679/14678} (cannot be {23678} because {23}{67}8 and [72638] clash with R4C1 and {26}[738] clashes with R4C56 + R5C6 = [378]), no 2
11e. R16C6 + R1C7 = 11 (step 6)
11f. 2 in C6 only in R16C6 -> R16C6 + R1C7 = {23}6/{26}3, clean-up: no 2 in R3C9 (step 3i)
11g. Naked pair {36} in R1C7 + R3C9, locked for N3
11h. 5 in C6 only in R789C6, locked for N8, clean-up: no 3 in R78C5
11i. 26(5) cage = {13679} only remaining combination, as follows:-
6 of {14678} must be in R23C6 (cannot be R4C68 or R4C78 = [68] which clash with R4C1)
{14678} cannot be {16}[748] which clashes with R4C56 + R5C67 = [3784]
{14678} cannot be {46}[178] because R23C6 + R4C56 + R5C6 = {14}[918] clashes with R78C6
{14678} cannot be [76148] which clashes with R4C56 + R5C67 = [9184]
[Cracked, the rest is straightforward.]

[From here onward clean-ups implied for 2-cell cages.]
12. 26(5) cage = {13679} -> R4C8 = 9, R6C7 = 8, R7C7 = 9, placed for D\
12a. R4C8 = R238C7 + 1 (step 5j), R4C8 = 9 -> R238C7 = 8 = {125} -> R23C7 = {12}, R3C8 = 7 (cage sum), R8C7 = 5 -> R8C8 = 3, placed for D\, R3C3 = 5, placed for D\, R3C4 = 8, R9C67 = [57]
12b. R5C7 = 4 (hidden single in C7) -> R5C6 = 8, R78C6 = [49]
12c. R9C5 = 8 (hidden single in C5) -> R7C8 + R8C9 = [68] (hidden pair in N9)
12d. 4 in N9 only in R9C89, locked for R9
12e. R23C2 = [84], R3C1 = 2 -> R4C1 = 8, R3C7 = 1, placed for D/, R2C7 = 2, R2C6 = 1 (hidden single in C6), R2C3 = 3, R34C5 = [93], R4C7 = 6, R4C6 = 7, placed for D/, R4C23 = [54]
12f. Naked pair {26} in R5C5 + R6C6, locked for N5 and D\ -> R4C4 = 1, placed for D\
12g. Naked pair {26} in R5C5 + R8C2, locked for D/
12h. R7C3 = 8 -> R6C3 + R7C4 = 5 = [23]
12i. R5C23 = [36]
12j. R8C2 + R9C1 = [63] (hidden pair on D/)
12k. R1C7 = 3, R34C9 = [62] -> R5C89 = 8 = [17]

and the rest is naked singles without using the diagonals.

I'll rate my walkthrough for A343X at least Hard 1.25 for my step 11. Some might think it should be higher, but one can do a lot of combination analysis for that rating.

Prelims courtesy of SudokuSolver
Cage 17(2) n69 - cells = {89}
Cage 8(2) n9 - cells do not use 489
Cage 8(2) n8 - cells do not use 489
Cage 12(2) n89 - cells do not use 126
Cage 12(2) n25 - cells do not use 126
Cage 12(2) n56 - cells do not use 126
Cage 12(2) n1 - cells do not use 126
Cage 13(2) n8 - cells do not use 123
Cage 13(2) n12 - cells do not use 123
Cage 9(2) n4 - cells do not use 9
Cage 9(2) n4 - cells do not use 9
Cage 10(2) n14 - cells do not use 5
Cage 10(3) n3 - cells do not use 89
Cage 26(4) n3 - cells do not use 1

No routine clean-up unless stated.
1. 17(2)r6c7 = {89} only: both locked for c7

2. "45" on n9: 3 innies r7c78+r9c7 = 22 = (589/679}(no 1,2,3,4)
2a. must have 5 or 7 which must be in r9c7 -> no 5,7 in r7c8
2b. must have 9: locked for r7 and n9

3. "45" on c89: 3 innies r348c8 = 19 (no 1)
3a. must have 8 or 9 which are only in r4c8 -> r4c8 = {89}
3b. can't have both 8 & 9 since both are only in r4c8-> = {379/469/478/568}(no 2)

4. "45" on n3: 2 innies r1c7+r3c9 = 9
4a. no 1,9 in r3c9

5. 1 in n3 only in c7: locked for c7
5a. no 7 in r8c8

6. "45" on n478: 2 outies r3c1+r9c7 = 9 = [45/27]
6a. r4c1 = {68}

7. "45" on n1: 3 innies r3c1+r23c3 = 10 and must have 2,4 for r3c1 = {127/145/235}(no 6,8,9)
7a. can't have two of 2,4 -> no 2,4 in r23c3
7b. can't have two of 5,7 -> r2c3 = {13}
7c. r3c34 = [58/76] = 5 in r3c3 or 6 in r3c4

8. "45" on n1: 2 innies r23c3 = 1 outie r4c1
8a. = [15][6]/[17/35][8] = 5 in r3c3 or 8 in r4c1 (no eliminations yet)

9. h19(3)r348c8 (step 3b) must have 3,5,6 for r8c8 = {379/469/568}
9a. but [685] blocked by r3c34 (by D\, step 7c)
9b. and [586] blocked by r3c3+r4c1 (step 8a)
9c. h19(3)r348c8 = {379/469}(no 5,8)
9d. r4c8 = 9, r38c8 = [73/46]

Pretty straightforward from here. The other walkthroughs show the key areas to finish it off.