I found a nice shortcut early in my solving path in step 2 which pretty well cracked it. This comment made here as I didn't want to post a spoiler. I hope that steps 2b and 2c are correct.
Prelims
a) R3C12 = {69/78}
b) R34C6 = {19/28/37/46}, no 5
c) R45C1 = {39/48/57}, no 1,2,6
d) R45C2 = {19/28/37/46}, no 5
e) R5C89 = {29/38/47/56}, no 1
f) R9C78 = {49/58/67}, no 1,2,3
g) 10(3) cage at R5C3 = {127/136/145/235}, no 8,9
h) 11(3) cage at R8C1 = {128/137/146/236/245}, no 9
i) 45(9) cage at R1C4 = {123456789}
1a. 45 rule on N1 1 innie R3C3 = 4, placed for D\, clean-up: no 6 in R4C6
1b. 45 rule on R9 2 innies R9C19 = 13 = [49]/{58/67}, no 1,2,3
1c. 45 rule on N9 2 outies R78C6 = 17 = {89}, locked for C6 and N8, clean-up: no 1,2 in R34C6
1d. 36(6) cage must contain 7,8,9 -> R6C5 + R8C3 = {89}, 7 locked for N8
1e. Naked pair {89} in R8C36, locked for R8
1f. 45 rule on R6789 2 innies R6C46 = 3 = {12}, locked for R6 and N5
1g. Min R56C4 = 4 -> max R5C3 = 6
1h. 45 rule on N36 3 innies R1C789 = 20 = {389/479/569/578}, no 1,2
1i. 45(9) cage at R1C4 = {123456789}, 1,2 locked for N2
1j. 4 in N2 only in R1C456 + R2C56 + R3C5, locked for 45(9) cage, no 4 in R1C789
1k. 45 rule on N236 3 innies R23C4 + R3C6 = 20 = {389/569/578}
[I initially saw this as in step 2a but decided to write the shorter version here.]
1l. R3C6 = {367} -> no 3,6,7 in R23C4
1m. 45 rule on N89 2 outies R6C5 + R8C3 = 3 innies R9C456 + 8
1n. R6C5 + R8C3 = 17 -> R9C456 = 9 = {126/135/234}
1o. Killer triple 4,5,6 in R9C19, R9C456 and R9C78, locked for R9
2a! Innies-outies for N2, R1C789 must contain exactly the same combination as R23C4 + R3C6
2b! R23C4 + R3C6 cannot be {569} = {59}6 which would require R3C456 = {59}6 (combo crossover clash because of R1C789 = {569}, locked for N3 and R3C12 = {78})
2c! R23C4 + R3C6 cannot be {578} = {58}7 which would require R3C456 = {58}7 (combo crossover clash because of R1C789 = {578}, locked for N3 and R3C12 = {69})
2d. -> R1C789 = {389}, locked for R1 and N3, R23C4 = {89}, locked for C4 and N2, R3C6 = 3, R3C7 = 7, placed for D/, clean-up: no 5 in R5C1, no 3 in R5C2, no 6 in R9C9 (step 1b)
2e. R3C3 = 4, R23C4 = {89} = 17 -> R4C34 = 7 = [16/25]
2f. 19(4) cage at R4C5 = {1369/1459/1468/2359/2368/2458}
2g, Killer pair 5,6 in R4C4 and 19(4) cage, locked for N5
2h. 10(3) cage at R5C3 = {136/145/235}
2i. 5,6 only in R5C3 -> R5C3 = {56}
2j. 19(4) cage = {1459/1468/2359/2368/2458} (cannot be {1369} which clashes with 10(3) cage)
2k. R5C89 = {29/38/47/56} (cannot be {56} which clashes with R5C3), no 5,6
3a. Hidden killer pair 1,2 in R45C2 and R4C3 for N4, R4C3 = {12} -> R45C2 must contain one of 1,2 = {19/28}, no 3,4,6,7
3b. 45 rule on R789 4 outies R6C1235 = 25 = {3589/3679/4579/4678}
3c. Killer pair {56} in R5C3 and R6C123, locked for N4, clean-up: no 7 in R5C1
3d. Killer pair 8,9 in R45C1 and R45C2, locked for N4
3e. R6C1235 = {3679/4579/4678} (cannot be {3589} because 8,9 only in R6C5), 7 locked for R6 and 32(6) cage at R6C1
3f. 32(6) cage = {145679/235679/245678} (cannot be {125789/134789} which clash with R8C3)
3g. Killer pair 8,9 in 32(6) cage and R8C3, locked for N7, clean-up: no 5 in R9C9 (step 1b)
3h. Killer pair 8,9 in 32(6) cage and R7C6, locked for R7
3i. 19(5) cage at R9C2 = {12367} (only remaining combination, cannot be {12457/13456} which clash with R9C78), 6,7 locked for R9, 7 locked for N7
3j. R6C123 = {367/457/467} -> R7C123 = {159/169/258/268/259}, no 3,4
3k. 4 in N7 only in 11(3) cage at R8C1 = {146/245}, no 3
3l. R9C23 = {37} (hidden pair in N7) -> R9C456 = {126}, locked for N8
3m. R7C123 = {169/258/259} (cannot be {159/268} which clash with 11(3) cage)
4a. 32(6) cage at R7C6 contains 8,9 = {125789/134789/135689/234689}
4b. Killer pair 4,5 in 32(6) cage and R9C78, locked for N9
4c. 17(3) cage at R8C6 = {179/278/368} (cannot be {269} which clashes with 11(3) cage at R8C1
4d. 32(6) cage = {125789/135689/234689} (cannot be {134789} which clashes with 17(3) cage)
4e. 11(3) cage at R8C1 (step 3k) = {146/245}, consider combinations for 32(6) cage
32(6) cage = {125789/135689} => 4 in N9 only in R9C78, locked for R9 => R9C1 = 5 => 11(3) cage = {245}
or 32(6) cage = {234689} => R8C78 = {17} (hidden pair in N9), 1 locked for R8 => 11(3) cage = {245}
-> 11(3) cage = {245}, 2,5 locked for N7
4f. R7C123 (step 3m) = {169} (only remaining combination), locked for R7, 9 locked for N7, 6 locked for 32(6) cage at R6C1 -> R8C3 = 8, R6C5 = 9, R78C6 = [89], R9C9 = 9, placed for D\, R9C1 = 4 (step 1b), placed for D/, clean-up: no 8 in R45C1, no 2 in R5C8
4g. Naked pair {25} in R8C12, locked for R8
4h. Naked pair {58} in R9C78, 5 locked for N9
4i. Naked pair {39} in R45C1, locked for C1 and N4, clean-up: no 6 in R3C2, no 1 in R45C2
4j. Naked pair {28} in R45C2, locked for C2, 2 locked for N4 -> R8C2 = 5, placed for D/, R4C3 = 1 -> R4C4 = 6 (step 2e), placed for D\, clean-up: no 7 in R3C1
4k. Naked triple {457} in R6C123, 4,5 locked for R6, 5 locked for N4, R5C3 = 6 -> R56C4 = 4 = [31], 1 placed for D/, R5C5 = 8, placed for D/, R45C1 = [39], R45C2 = [82], R6C6 = 2, placed for D\, naked pair {47} in R5C89, locked for N6, 4 locked for R5 -> R5C6 = 5, R4C5 = 4
4l. Naked pair {26} in R2C8 + R3C7, locked for N3
4m. 12(3) cage at R2C8 = {156} (only remaining combination, cannot be {147} because R2C8 only contains 2,6, cannot be {246} because 2,6 only in R2C8) -> R2C8 = 6, R23C9 = {15}, locked for C9 and N3
4n. R3C8 = 7, R4C9 = 2 -> R4C8 = 5 (cage sum), R3C2 = 9 -> R3C1 = 6
4o. R1C1 = 5 (hidden single on D\), R2C1 = 8 (hidden single in C1) -> R1C2 = 1 (cage sum)
4p. R8C8 = 1 (hidden single on D\), R8C6 = 9 -> R8C7 = 7, R7C7 = 3, placed for D\
and the rest is naked singles without using the diagonals.
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