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Thanks Ed for a couple of corrections

Prelims

a) R3C56 = {19/28/37/46}, no 5
b) R56C1 = {49/58/67}, no 1,2,3
c) R89C6 = {39/48/57}, no 1,2,6
d) R89C7 = {17/26/35}, no 4,8,9
e) R8C89 = {19/28/37/46}, no 5
f) R9C89 = {16/25/34}, no 7,8,9
g) 20(3) cage at R2C1 = {389/479/569/578}, no 1,2
h) 11(3) cage at R3C7 = {128/137/146/236/245}, no 9
i) 11(3) cage at R3C8 = {128/137/146/236/245}, no 9
j) 9(3) cage at R6C5 = {126/135/234}, no 7,8,9
k) 22(3) cage at R6C8 = {589/679}
l) 9(3) cage at R7C1 = {126/135/234}, no 7,8,9
m) 13(4) cage at R1C3 = {1237/1246/1345}, no 8,9
n) 27(4) cage at R1C9 = {3789/4689/5679}, no 1,2
o) 27(4) cage at R7C3 = {3789/4689/5679}, no 1,2

Steps resulting from Prelims
1a. 13(4) cage at R1C3 = {1237/1246/1345}, 1 locked for C3
1b. 22(3) cage at R6C8 = {589/679}, 9 locked for NR3C9
1c. 27(4) cage at R7C3 = {3789/4689/5679}, CPE no 9 in R6C4

2. 45 rule on R1234 1 innie R4C9 = 1 outie R5C8 + 6, R4C9 = {78}, R5C8 = {12}
2a. Killer pair 7,8 in R4C9 and 22(3) cage, locked for NR3C9, clean-up: no 1 in R9C7
2b. 4 in NR3C9 only in R35C9 + R6C7, CPE no 4 in R6C9
2c. 27(4) cage at R1C9 = {3789/4689/5679}, 9 locked for NR1C6
2d. 3 of {3789} must be in R3C9 -> no 3 in R1C9 + R2C89

3. 45 rule on C1 4 innies R1789C1 = 12 = {1236/1245}, no 7,8,9
3a. 20(3) cage at R2C1 = {389/479/578} (cannot be {569} which clashes with R1789C1), no 6

4. 45 rule on NR5C3 2 outies R7C6 + R9C4 = 15 = {69/78}
4a. Min R7C6 = 6 -> max R789C5 = 10, no 8,9 in R789C5

5. Law of Leftovers (LoL) for C1234 three outies R789C5 must exactly equal three innies R345C4, no 8,9 in R789C5 -> no 8,9 in R345C4

6. LoL for C789 three outies R189C6 must exactly equal three innies R345C7
6a. R89C6 = 12 -> two of the cells in R345C7 must total 12, cannot be R34C7 because they are part of 11(3) cage at R3C7) -> R5C7 must be part of this hidden 12(2) cage, no 1,2,6 in R5C7

7. 45 rule on NR3C9 + NR6C9 3 innies R3C9 + R5C8 + R6C7 = 11 = {146/236/245}
7a. R5C8 = {12} -> no 1,2 in R6C7
7b. Killer pair 5,6 in R3C9 + R6C7 and 22(3) cage at R6C8, locked for NR3C9, clean-up: no 2,3 in R9C7

8. 45 rule on C12 2 outies R89C3 = 14 = {59/68}
8a. 45 rule on C12 2 innies R9C12 = 9 = [18/27]/{36/45}, no 1,2,9 in R9C2

9. 45 rule on C123 4 outies R6789C4 = 26 = {2789/3689/4589/4679/5678}, no 1
9a. 1 in NR5C3 only in R789C5, locked for C5,clean-up: no 9 in R3C6
9b. LoL (step 5), 1 in NR5C3 only in R789C5 -> 1 in R345C4, locked for C4 and NR3C4

10. 9(3) cage at R6C5 = {234} (only remaining combination), locked for R6, 2 also locked for NR3C4, clean-up: no 9 in R5C1
10a. R3C9 + R5C8 + R6C7 (step 7) = {146/236/245}
10b. 5,6 only in R3C9 -> R3C9 = {56}

11. LoL (step 5) no 2 in R345C4 -> no 2 in R789C5
11a. 16(4) cage at R7C5 = {1348/1357/1456}, no 9, clean-up: no 6 in R9C4 (step 4)
11b. R7C6 = {678} -> no 6,7 in R789C5
11c. LoL (step 5), no 6,7 in R789C5 -> no 6,7 in R345C4

12. R5C3 = 2 (hidden single in NR5C3), R5C8 = 1, R4C9 = 7 (step 2), both placed for NR3C9, clean-up: no 6 in 22(3) cage at R6C8, no 3 in R8C8, no 9 in R8C9. no 7 in R9C7, no 6 in R9C9
12a. R5C8 = 1 -> R34C8 = 10 = {28/46}/[73], no 5, no 3 in R3C8
12b. R3C9 = 6 (hidden single in NR3C9), clean-up: no 4 in R3C56, no 4 in R4C8, no 4 in R8C8
12c. R8C7 = 2 (hidden single in NR3C9), R9C7 = 6, placed for NR6C9, clean-up: no 8 in R8C3 (step 8), no 8 in R8C8, no 4,8 in R8C9, no 1 in R9C9

13. Naked quint {12345} in R345C4 + R6C56, locked for NR3C4, 5 also locked for C4
13a. LoL (step 5), 5 in R345C4 -> 5 in R789C5, locked for C5 and NR5C3
13b. 1 in C4 only in R34C4 -> 14(3) cage at R3C4 = {149/158}, no 3,6

14. 13(4) cage at R1C3 = {1345} (only remaining combination), locked for C3, clean-up: no 9 in R89C3 (step 8)
14a. R89C3 = [68], placed for NR6C2, clean-up: no 7 in R7C6 (step 4), no 4 in R8C6, no 1,3 in R9C1 (step 8a), no 3 in R9C2 (step 8a)
14b. Naked pair {79} in R67C3, locked for NR5C3
14c. R7C3 + R9C4 = {79} = 16 -> R78C4 = 11 = {38}, locked for C4 and NR5C3 -> R6C4 = 6, R6C3 = 9 (cage sum), R7C3 = 7, R9C4 = 9, placed for NR6C2, R7C6 = 6 (step 4), placed for NR3C4, clean-up: no 4,7 in R5C1, no 3 in R8C6

15. Naked triple {145} in R345C4, locked for C4 and NR3C4
15a. Naked pair {23} in R6C56 -> R6C7 = 4, placed for NR3C9, R5C9 = 3, R8C9 = 1, R8C8 = 9, placed for NR6C9, clean-up: no 3 in R9C6, no 4 in R9C8
15b. Naked triple {145} in R789C5, locked for C5

16. Naked pair {27} in R12C4, locked for NR1C1, R2C5 = 6 (cage sum)

17. 25(4) cage at R5C4 = {4579} (only remaining combination) -> R5C4 = 4, R5C7 = 5, placed for NR1C5, R5C56 = {79}, locked for R5 and NR3C4, R4C5 = 8, clean-up: no 2 in R3C6, no 2 in R3C8 (step 12a), no 8 in R6C1

18. 20(3) cage at R2C1 (step 3a) = {389} (only remaining combination), locked for C1 and NR1C1 -> R5C1 = 6, R6C1 = 7, both placed for NR3C2, R5C2 = 8, R67C2 = 5 = [14], placed for NR6C2, R2C2 = 5, placed for NR1C1, R1C2 = 6, R8C12 = [53], R7C1 = 1 (cage sum), R1C13 = [41], R9C12 = [27], R78C4 = [38], R8C6 = 7, R9C6 = 5, placed for NR5C9, R6C9 = 8, R9C89 = [34], R12C9 = [59], R2C8 = 7 (cage sum), placed for NR1C6, R67C8 = [58], R1C8 = 2, R12C4 = [72]

19. Naked pair {38} in R1C67, locked for R1 and 27(6) cage at R1C5 -> R1C5 = 9, R2C67 = [41]

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

One surprising thing about my solving path is that, although there are many LoLs available for the columns, I only used two of them, one of which I used repeatedly.