Thanks Ed for your comment about step 7, which I've re-phrased for clarity, and to Afmob for correcting a couple of typos and pointing out the easy step which I missed (see note after step 12b); if I'd spotted it the rest of my solving path would have been a lot easier.
Prelims
a) R1C34 = {49/58/67}, no 1,2,3
b) R1C56 = {16/25/34}, no 7,8,9
c) R12C7 = {19/28/37/46}, no 5
d) R2C45 = {16/25/34}, no 7,8,9
e) R34C1 = {16/25/34}, no 7,8,9
f) R45C2 = {18/27/36/45}, no 9
g) R56C1 = {17/26/35}, no 4,8,9
h) R7C12 = {16/25/34}, no 7,8,9
i) 19(3) cage at R8C1 = {289/379/469/478/568}, no 1
j) 32(5) cage at R6C3 = {26789/35789/45689}, no 1
1. 45 rule on N3 1 innie R3C7 = 6, placed for D/, clean-up: no 4 in R12C7, no 1 in R4C1
1a. 30(5) cage at R2C6 contains 6 = {25689/34689/35679/45678}, no 1
2. 45 rule on N7 1 innie R7C3 = 3, placed for D/, clean-up: no 4 in R7C12
2a. 32(5) cage at R6C3 contains 3 = {35789} (only remaining combination), no 2,4,6
3. 19(3) cage at R8C1 = {289/469/478} (cannot be {568} which clashes with R7C12), no 5
4. Killer quint 5,6,7,8,9 in R45C2, R56C1 and 32(5) cage at R6C3, locked for N4, clean-up: no 1,2 in R3C1
4a. 9 in N4 only in R5C3 + R6C23, locked for 32(5) cage, no 9 in R6C4
5. 45 rule on N1 2 innies R1C3 + R3C1 = 12 = [75/84/93] -> R1C4 = {456}
6. Killer triple 4,5,6 in R1C4 + R1C56 + R2C45, locked for N2
7. 30(5) cage at R2C6 (step 1a) = {25689/34689/35679} (cannot be {45678} because 4,5 only in R4C6) -> R4C6 = {45} (only possible place for 4 or 5), 9 locked for N2
7a. Killer triple 1,2,3 in R1C56, R2C45 and 30(5) cage, locked for N2
[Alternatively hidden killer pair 7,8 in R3C4 and 30(5) cage for N2, 30(5) cage contains one of 7,8 -> R3C4 = {78}.]
8. 45 rule on C89 2 outies R49C7 = 9 = {18/27/45}, no 3,9
[At this stage I saw locking-out cages in N4 using places for 6, but after the next few steps it’s not needed although the placements for 6 in N4 was useful later as part of my key breakthrough.]
9. 45 rule on C123 2 outies R16C4 = 1 innie R4C3 + 9
9a. R16C4 cannot total 10 -> no 1 in R4C3
10. 45 rule on N47 1 outie R6C4 = 2 innies R4C13 + 1
10a. Min R4C13 = 5 -> no 5 in R6C4
10b. R6C4 = {78} -> R4C13 = 6,7 = {24}/[34], 4 locked for R4 and N4 -> R4C6 = 5, placed for D/, clean-up: no 2 in R1C5, no 5 in R5C2, no 4,5 in R9C7 (step 8)
11. Naked pair {78} in R36C4, locked for C4
11a. Naked pair {78} in R36C4, CPE no 7,8 in R4C5
12. 45 rule on N2 2 remaining innies R13C4 = 12 = [48/57], no 6, clean-up: no 7 in R1C3, no 5 in R3C1 (step 5) -> no 2 in R4C1
12a. Naked pair {34} in R34C1, locked for C1, clean-up: no 5 in R56C1
12b. R45C2 = {18/36} (cannot be {27} which clashes with R56C1), no 2,7 in R45C2
[Afmob pointed out that after this I missed 7 in R4 only in R4C789, locked for N6 -> 24(4) cage at R4C7 must contain 7 so cannot contain 1.]
13. R12C7 = {19/37} (cannot be {28} which clashes with R49C7), no 2,8 in R12C7
13a. Killer pair 1,7 in R12C7 and R49C7, locked for C7
14. 28(5) cage at R3C4 = {14689/23689/24679} (cannot be {13789} because R4C3 only contains 2,4, cannot be {34678} because 7,8 only in R3C4), 6,9 locked for N5
15. 15(4) cage at R5C5 = {1248/1347}, 1,4 locked for N5
16. 45 rule on N2 1 remaining outie R1C3 = 1 innie R3C4 + 1 -> R1C3 + R3C4 = [87/98], CPE no 8 in R3C23
17. 45 rule on N1 1 outie R1C4 = 1 innie R3C1 + 1 -> R1C4 + R3C1 = [43/54], CPE no 4 in R1C2
18. 45 rule on N8 2 innies R7C56 = 1 outie R8C7 + 9
18a. Max R7C56 = 17 -> max R8C7 = 8
19. 45 rule on N89 4 innies R7C5678 = 25 = {1789/4579/4678} (cannot be {2689} which clashes with R7C12), no 2, 7 locked for R7
19a. 5 must be in R7C12 + R7C5678, locked for R7
20. 17(3) cage at R1C1 = {179/269/278/359} (cannot be {368} which clashes with R1C3 + R3C1)
20a. 3 of {359} must be in R1C2 -> no 5 in R1C2
20b. Killer pair 8,9 in 17(3) cage and R1C3, locked for N1
[I’m struggling to make progress, so I’ll try a short forcing chain.]
21. Consider combinations for 19(3) cage at R8C1 (step 3) = {289/469/478}
19(3) cage = {289} => R7C12 = {16}, killer pair 1,6 in R56C1 and R7C1, locked for C1
or 19(3) cage = {469/478} => killer pair 6,7 in R56C1 and 19(3) cage, locked for C1
-> no 6 in R12C1
21a. 17(3) cage at R1C1 (step 20) = {179/278/359} (cannot be {269} which clashes with R45C2 + R56C1 which must contain 6 in C2 or {26} in C1), no 6
[I first saw this as 17(3) cage at R1C1 (step 20) = {179/278/359} (cannot be {269} which clashes with 19(3) cage = {289/469} and with 19(3) cage = {478} + R56C1), no 6]
22. 6 in R1 only in R2C23 = {16}, locked for R1 and N2, clean-up: no 9 in R2C7
23. 15(3) cage at R1C8 = {249/258/348/357} (cannot be {159} which clashes with R1C34), no 1
24. R5C5 = 1 (hidden single on D/), placed for D\, R1C56 = [61], clean-up: no 8 in R4C2, no 7 in R6C1
24a. 6 in N5 only in R45C4, locked for C4
25. 1 in N8 only in 19(4) cage at R7C4 = {1279/1459} (cannot be {1378} because 7,8 only in R9C5), no 3,8, 9 locked for N8
25a. 7 of {1279} must be in R9C5 -> no 2 in R9C5
25b. R7C56 = R8C7 + 9 (step 18)
25c. Max R7C56 = 15 -> no 8 in R8C7
26. 3 in N8 only in R8C56 + R9C6, locked for 17(4) cage at R8C5, no 3 in R8C7
26a. 17(4) cage = {2348/2357}, no 6
26b. 17(4) cage = {2348/2357}, CPE no 2 in R8C4
27. R7C6 = 6 (hidden single in N8), clean-up: no 1 in R7C12
27a. Naked pair {25} in R7C12, locked for R7 and N7
27b. R7C5678 (step 19) contains 6 = {4678} (only remaining combination), locked for R7
28. 19(3) cage at R8C1 (step 3) = {469/478} -> R8C2 = 4, placed for D/
29. 2 on D/ only in 15(3) cage at R1C8 (step 23) = {249/258}, no 3,7, 2 locked for N3
29a. 4,5 only in R1C8 -> R1C8 = {45}
29b. Naked pair {45} in R1C48, locked for R1
30. 4 in C7 only in R567C7, locked for 29(5) cage at R5C7, no 4 in R7C5
30a. 29(5) cage contains 4,6 = {24689} (only remaining combination, cannot be {34679} which clashes with R12C7) -> R7C5 = 8, R7C7 = 4, placed for D\, R56C7 = {29}, locked for C7 and N6, clean-up: no 1 in R2C7, no 7 in R49C7 (step 8)
30b. Naked pair {37} in R12C7, locked for N3
31. R8C7 = 5 -> 17(4) cage at R8C5 = {2357}, 2,7 locked for N8
31a. Naked pair {19} in R78C4, locked for C4 and N8
31b. Naked pair {45} in R19C4, locked for C4, clean-up: no 2,3 in R2C5
31c. R4C5 = 9 (hidden single in N5)
32. R7C8 = 7 -> 17(4) cage at R5C8 = {1367/1457}, no 8, 1 locked for R6 and N6 -> R49C7 = [81], R7C9 = 9, R78C4 = [19], clean-up: no 7 in R5C1
33. Naked pair {26} in R56C1, locked for C1 and N4 -> R4C3 = 4, R34C1 = [43]
34. R2C1 = 1 (hidden single in C1), R1C1= 9 (hidden single on D\), R1C2 = 7 (cage sum), R1C3 = 8 -> R1C4 = 5, R1C89 = [42], R2C2 = 9 (cage sum), R2C5 = 4 -> R2C4 = 3
35. R6C6 = 7 (hidden single on D\), R6C4 = 8, placed for D/, R89C1 = [87]
36. R4C789 = [867], R5C9 = 3 (cage sum)
37. R8C8 = 3 (hidden single on D\)
and the rest is naked singles, without using the diagonals.