1. Naked sextet (!) r89c789 = {345789}
-> r7c789 = {126}
-> r7c6 = 3
-> 12(2)s in n8 are {48} and {57}
-> whichever of {48} or {57} is in r89c6 is also in r7c23
-> NS r7c1 = 9
-> 9 in n8 in r89c5

2. Since one of (45) is in r89c6 and also in r7c23 -> 12(4)r8c3 cannot be {1245}
-> 12(4)r8c3 = {1236} with 3 in r89c3
-> 14(3)n7 = [9{14}]
-> 12(2)r7c2 = {57}
-> 12(2)r7c4 = {48}
-> 12(2)r8c6 = {57}
-> 8 in n7 in r8c1 or r9c2

3! Since 4 and 8 are on different rows in n9 -> 4 and 8 cannot both be in the same row in n7
-> either r89c1 is [84] or r89c2 is [48]
-> 12(2)s in n1 cannot be {48}
-> 12(2)s in n1 are {57} and [39]
-> 12(2)r4c1 = {48}
-> r89c2 = [48]
-> r9c1 = 1
-> r8c4 = 1

4. Whichever of (57) is in r12c2 must go in n7 in r7c3
-> it must also go in c1/n4 in r6c1
-> NP r38c1 = {26}

5. Given:
a) One of (26) is in r89c3 and also in r3c1, and
b) 3 in n1 in r12c1 and 3 in n7 in r89c3 -> 3 in c2/n4 in r456c2
-> 12(4)r3c2 cannot be {1236}
(3 would have to go in r4c2 which leaves no place for the (2 or 6) that's in r3c1 and r89c3)
-> 12(4)r3c2 = {1245}
-> r3c3 = 4
Also -> 5 in r4c23
-> 5 in n1 in r12c1
-> 7 in n1 in r12c2
-> r7c23 = [57]
-> r4c3 = 5
-> r34c2 = {12}
Also -> r6c1 = 7
Also -> r56c2 = {36}

6. Innies - outies n6 -> r45c9 = r3c7 + 9
r3c7 from (2356) (1,4 already in D/)
r3c7 cannot be 5 (would put r45c9 = +14(2) which leaves no place for 7 in n6)
-> r3c7 from (236)
a) 2 in r3c7 puts r45c9 = {29}
b) 3 in r3c7 puts either 12(2)r8c8 = {39} or 12(2)r8c9 = {39}
puts 12(2)r6c8 not = {39}
puts r45c9 = {39}
c) 6 in r3c7 puts r45c9 = {69}

-> r45c9 = {(2|3|6)9}
-> HS 7 in n6 -> 12(2)r5c7 = [75]
-> 12(2)r6c8 = {48}
-> 12(2)r8c7 = [84]
-> 12(2)r8c8 = {39}
-> 12(2)r8c9 = {57}

7. Naked Quad in D\ in r1289 = {3579}
-> 8 in D\ in r4c4 or r5c5
Given that and also given:
a) r45c1 = {48}, and
b) r7c45 = {48}, and
c) r6c89 = {48}, and
d) 4 already in both diagonals
-> 4 and 8 in n5 are in not in the same row or column.
-> Only places for them in n5 are r4c4 = 8 and r5c6 = 4
-> r45c1 = [48], r7c45 = [48]
-> 4 in n2 in r23c5 and 8 in n2 in r123c6

8. -> HS 7 in n5 -> r4c5 = 7
Also 8 in D/ in r1c9 or r2c8
-> HS 8 in n2/r3 -> r3c6 = 8
Also 9 in n8 in r89c5
Also 9 in n3 in r12c7
-> HS 9 in n2/r3 -> r3c4 = 9
-> HS 9 in D/ -> r4c6 = 9
-> HS 9 in n6 -> r5c9 = 9
-> HS 9 in n4/c3 -> r6c3 = 9
Also HS 5 in n5/r5 -> r5c4 = 5 (cannot be in r5c5 since NQ in D\ r1289 = {3579})
-> NS 5 in D/ -> r2c8 = 5
-> r1c12 = [57]
-> r2c12 = [39]
-> r89c8 = [39]
-> r89c9 = [57]
-> r89c6 = [75]
Also r9c3 = 3

9. Finally
HS 8 in D/ -> r1c9 = 8
-> r2c3 = 8
Also HS 5 in n2/r3 -> r3c5 = 5
Also 3 in n5 in r6c45
-> HS 3 in n4/c2 -> r5c2 = 3
-> r6c2 = 6
-> r5c5 = 6
-> r9c45 = [62]
-> r8c3 = 2
-> r38c1 = [26]
-> r34c2 = [12]
etc.

1.25. The (first) key move was my Step 3.

even though it took me a very long time to spot the most important one. An ideal puzzle for wellbeback who spotted that a lot quicker than I did.
It felt like a Human Solvable; maybe it would have been if the SS score had been higher. It is, of course, also acceptable as an Assassin.

Prelims

a) R1C12 = {39/47/57}, no 1,2,6
b) R2C12 = {39/47/57}, no 1,2,6
c) R45C1 = {39/47/57}, no 1,2,6
d) R56C7 = {39/47/57}, no 1,2,6
e) R6C89 = {39/47/57}, no 1,2,6
f) R7C23 = {39/47/57}, no 1,2,6
g) R7C45 = {39/47/57}, no 1,2,6
h) R89C6 = {39/47/57}, no 1,2,6
i) R89C7 = {39/47/57}, no 1,2,6
j) R89C7 = {39/47/57}, no 1,2,6
k) R89C7 = {39/47/57}, no 1,2,6
l) 12(4) cage at R3C2 = {1236/1245}
m) 12(4) cage at R3C7 = {1236/1245}
n) 12(4) cage at R7C6 = {1236/1245}
o) 12(4) cage at R8C3 = {1236/1245}

1. 45 rule on N9 1 outies R7C6 = 3, clean-up: no 9 in R7C2345, no 9 in R89C6
1a. Naked quad {4578} in R7C45 + R89C6, locked for N8
1b. R7C1 = 9 (hidden single in R7) -> R8C2 + R9C1 = 5 = {14/23}, clean-up: no 3 in R12C2, no 3 in R45C1
1c. 9 in N8 only in R89C5, locked for C5
1d. 12(4) cage at R8C3 = {1236} (cannot be {1245} which clashes with R7C23), 3 locked for C3 and N7, clean-up: no 2 in R8C2 + R9C1
1e. Naked pair {14} in R8C2 + R9C1, locked for N7 and D/, clean-up: no 8 in R7C23
1f. Naked pair {57} in R7C23, locked for R7 and N7
1g. Naked pair {48} in R7C45, locked for R7 and N8
1h. Naked pair {57} in R89C6, locked for C6
1i. 12(4) cage at R8C3 = {1236}, 1 locked for C4 and N8

2. 12(4) cage at R3C7 = {1236/1245}, 1 locked for N6
2a. 5 of {1245} must be in R3C7 (cannot be 2{145} which clashes with R56C7 + R6C89) -> no 5 in R4C78 + R5C8

3. R89C8 and R89C9 must be different from R6C89 -> whichever combination is in R6C89 must be in R89C7
3a. 12(4) cage at R3C7 = {1236/1245}
3b. Consider combinations for R6C89 = {39/48/57}
R6C89 = {39}, locked for N6 => R56C7 = {48/57} => 12(4) cage = {1236}
or R6C89 = {48}, locked for N6 => 12(4) cage = {1236}
or R6C89 = {57} => R89C7 = {57}, locked for C7 => 12(4) cage = {1236}
-> 12(4) cage = {1236}
3c. R56C7 = {48/57} (cannot be {39} which clashes with 12(4) cage)
3d. R6C89 = {48/57} (cannot be {39} because R6C89 {39} + R89C7 = {39} clash with 12(4) cage)
3e. R6C89 = {48/57} -> R89C7 = {48/57}
3f. Naked quad {4578} in R56C7 + R6C89, locked for N6
3g. Naked quad {4578} in R5689C7, locked for C7
3h. 9 in N6 only in R45C9, locked for C9, clean-up: no 3 in R89C9
3i. R89C8 = {39} (hidden pair in N9), locked for C8
3j. 12(4) cage = {1236}, 3 locked for C7
3k. Naked triple {126} in R457C8, locked for C8
3l. 9 on D/ only in R4C6 + R6C4, locked for N5

[It seems likely that 5,7 in N9 must be in R89C9, since R89C6 and R89C7 both {57} would appear to be a Killer UR. I’ll leave that for now and only use it if needed. I prefer puzzles which can be solved uniquely, rather than relying on uniqueness to solve them.]

4. In the same way as for step 3, R1C12, R2C12 and R45C1, which are all 12(2) cages and ‘see’ at least one cell of each other, must have different combinations -> one of R1C12 and R2C12 must be {39}, locked for N1
4a. Either R1C1 = 3 or R2C2 = 9 -> killer pair 3,9 in R1C1 + R2C2 and R8C8, locked for D\
4b. 3 in N1 only in R12C1, locked for C1
4c. 9 in N1 only in R12C2, locked for C2
4d. 9 in R3 only in R3C46, locked for N2

5. Taking step 4 a bit further, consider combinations for R45C1 = {48/57}
R45C1 = {48} => R1C12 and R2C12 = {39/57} => killer pair 5,7 in R12C2 and R7C2, locked for C2
or R45C1 = {57}, locked for N4
-> no 5,7 in R456C2
5a. R45C1 = {48} => R1C12 and R2C12 = {39/57}, 5,7 locked for N1 => hidden killer pair 5,7 in R12C1 and R6C1
or R45C1 = {57}, locked for N4
-> 5,7 in R12456C1, locked for C1

5. R1C12 + R2C12 + R45C1 must have different combinations (step 4) -> one of them must be {48}
5a. Grouped X-Wing for 4 in R1C12 + R2C12 + R45C1 and R8C2 + R9C1, no other 4 in C12
5b. Grouped X-Wing for 8 in R1C12 + R2C12 + R45C1 and R8C1 + R9C2, no other 8 in C12

6. Consider combinations for R45C1 = {48/57}
R45C1 = {48} => R1C12 and R2C12 = {39/57} => hidden killer pair 5,7 in R12C1 and R6C1 => R6C1 = {57} => R6C89 = {48} => R89C9 = {57}, killer pair 5,7 in R1C1 + R2C2 and R9C9, locked for D\ => R4C35 = [57] (hidden pair in R4)
or R45C1 = {57} => R45C2 + R6C12 = {1236} => R4C3 = 4
-> R4C3 = {45}
6a. R4C3 = {45} -> 12(4) cage at R3C2 = {1245}, 4 locked for C3
6b. Killer pair {45} in R45C1 and R4C3, locked for N4
6c. Caged X-Wing for 5 in 12(4) cage and R7C23, no other 5 in C23, clean-up: no 7 in R12C1
6d. 7 in C1 only in R456C1, locked for N4
6e. R45C1 = {48} => R4C3 = 5
or R45C1 = {57}, locked for C1, no 7 in R12C2 => 7 in N1 only in R12C3, locked for C3 => R7C3 = 5
-> no 5 in R3C3
6f. R45C1 = {48} => R4C3 = 5, R34C2 = {12} => R56C2 = {36}, locked for N4
or R45C1 = {57} => R45C2 + R6C12 = {1236}, locked for N4
-> no 6 in R56C3

7. Consider combinations for R89C9 = {48/57}
R89C9 = {48} => R6C89 = {57} => 7 in N4 only in R45C1 = {57} => R12C12 = {39/48} => killer pair 4,8 in R1C1 + R2C2 and R9C9, locked for D\
or R89C9 = {57} => R6C89 = {48}, locked for R6
-> no 4,8 in R6C6

[I should have spotted this earlier …; with hindsight it could have been done after step 1.]
8. R8C12 cannot be [84] which clashes with {48} in R89C7 or R89C9, R9C12 cannot be [48] which clashes with {48} in R89C7 or R89C9, R89C1 cannot be [84] which clashes with {48} in R1C12, R2C12 or R45C1 -> R89C2 = [48], R9C1 = 1, clean-up: no 4,8 in R12C1
8a. R45C1 = {48} (hidden pair in C1), locked for N4, R4C3 = 5, R7C3 = 7, placed for D/
8b. Naked pair {12} in R34C2, locked for C2 and 12(4) cage at R3C2 -> R3C3 = 4, placed for D\, clean-up: no 8 in R8C9
[So Killer UR wasn’t necessary ]
8c. Naked pair {57} in R89C9, locked for C9 and N9 -> R89C7 = [84], clean-up: no 5,7 in R6C8
8d. Killer pair 5,7 in R1C1 + R2C2 and R9C9, locked for D\
8e. 8 on D\ only in R4C4 + R5C5, locked for N5
8f. R6C1 = 7 (hidden single in C1) -> R56C7 = [75]
8g. Naked pair {48} in R6C89, locked for R6

9. R2C8 = 5 (hidden single on D/), R2C1 = 3 -> R2C2 = 9, placed for D\, R1C1 = 5, placed for D\, R1C2 = 7
9a. R3C8 = 7 (hidden single in C8)
9b. R4C5 = 7 (hidden single in R4)
9c. R2C4 = 7 (hidden single in C4)
9d. R5C4 = 5 (hidden single in N5)
9e. R5C6 = 4 (hidden single in N5), R45C1 = [48]
9f. R4C4 = 8 (hidden single in N5) -> R7C45 = [48]
9g. R1C9 = 8 (hidden single of D/) -> R1C8 = 4
9h. R23C5 = [45] (hidden pair in C5)

10. R3C46 = [98] (hidden pair in R3)
10a. R4C6 = 9 (hidden single in N5)
10b. R5C9 = 9 (hidden single in N6)
10c. R5C2 = 3 (hidden single in R5), R6C2 = 6
10d. R5C5 = 6 (hidden single in N5), placed for both diagonals
10e. Naked pair {29} in R89C5, locked for C5 and N8 -> R89C4 = [16]
10f. Naked pair {23} in R89C3, locked for C3 and N7
10g. R5C3 = 1, R5C8 = 2, R3C7 = 3, placed for D/, R6C4 = 2, R6C6 = 1, placed for D\

and the rest is naked singles, without using the diagonals.

I'll rate my walkthrough for A332X at least Hard 1.5.
I'm not sure why the SS score is only 1.5. However I noticed from the SS scoring summary that it used UR shortcuts.