A151 V2 was then next one of my backlog of unfinished puzzles that I went back to.
In Afmob's walkthrough steps 6b and 7d were neat while in manu's walkthrough the key step 3f was similar to my final breakthrough; I liked the way manu expressed it that the 3-cell cages in C1 and C9 cannot both contain 4.
Even though my solving path is similar to manu's, I decided to post my walkthrough because of the interesting step 31.
Here is my walkthrough for A151 V2. At the time this puzzle was originally active I'd got as far as step 24. When I resumed I did a bit of nibbling and then made real progress from step 28 onward.
Prelims
a) R1C456 = {126/135/234}, no 7,8,9
b) 21(3) cage at R3C2 = {489/579/678}, no 1,2,3
c) R456C1 = {126/135/234}, no 7,8,9
d) 9(3) cage at R6C4 = {126/135/234}, no 7,8,9
e) R9C456 = {127/136/145/235}, no 8,9
f) 28(4) cage at R1C7 = {4789/5689}, no 1,2,3
1. 45 rule on complete grid 1 innie R5C5 = 9, placed for D/ and D\
2. 45 rule on R1234 2 innies R4C19 = 6 = {15/24}
3. 45 rule on R5 2 remaining innies R5C19 = 8 = [17]/{26/35}, no 4,8, no 1 in R5C9
4. 45 rule on R6789 2 innies R6C19 = 12 = [39/48/57]
5. R456C1 = {135/234} (cannot be {126} because R6C1 only contains 3,4,5}, no 6, 3 locked in R56C1, locked for C1 and N4, clean-up: no 2 in R5C9 (step 3)
6. 45 rule on C1 2 outies R19C2 = 10 = {19/28/37/46}, no 5
7. 45 rule on C9 2 outies R19C8 = 12 = {39/48/57}, no 1,2,6
8. 45 rule on C1234 2 innies R19C4 = 9 = [27]/{36/45}, no 1, no 2 in R9C4
9. 45 rule on C5 2 remaining innies R19C5 = 3 = {12}, locked for C5
10. 45 rule on C6789 2 innies R19C6 = 7 = {16/25/34}, no 7
11. 45 rule on D/ 2 remaining innies R1C9 + R9C1 = 15 = {78}, locked for D/, CPE no 7,8 in R1C1 + R9C9
12. 45 rule on D\ 2 remaining innies R1C1 + R9C9 = 10 = {46}, locked for D\
13. R1C9 = {78} -> 15(4) cage in N3 = {1248/1257/1347} (other combinations don’t contain 7 or 8), no 6,9, 1 locked in R23C9, locked for C9 and N3, clean-up: no 5 in R4C1 (step 2), no 3 in R9C8 (step 7)
13a. R1C9 = {78} -> no 7,8 in R1C8 + R23C9, clean-up: no 4,5 in R9C8 (step 7)
13b 5 of {1257} must be in R1C8 -> no 5 in R23C9
14. R456C9 = {269/359/458} (cannot be {278} which clashes with R1C9, cannot be {368} because R4C9 only contains 2,4,5, cannot be {467} which clashes with R9C9), no 7, clean-up: no 1 in R5C1 (step 3), no 5 in R6C1 (step 4)
15. 12(3) cage at R6C6 = {138/237}, no 5, 3 locked for D\
16. R5C234 = {138/147/246} (cannot be {156/237} which clash with R5C19, cannot be {345} because 3 only in R5C4 and R5C23 = {45} clashes with R456C1), no 5
16a. 3 of {138} must be in R5C4 -> no 8 in R5C4
17. 45 rule on N4 4 innies R46C23 = 1 outie R5C4 + 24
17a. Max R46C23 = 30 -> max R5C4 = 6
17b. Min R46C23 = 25 (when R5C4 = 1) but cannot be {1789} which clashes with R5C234 = {47}1 -> no 1 in R6C23
18. Interactions between R1C456 and R9C456 using steps 8,9,10
18a. R1C456 cannot be [315/513] because R9C456 cannot be [622/424] -> R1C456 = {126/234}, no 5, 2 locked for R1 and N2, clean-up: no 8 in R9C2 (step 6), no 4 in R9C4 (step 8), no 2 in R9C6 (step 10)
18b. Killer pair 4,6 in R1C1 and R1C46, locked for R1, clean-up: no 4,6 in R9C2 (step 6), no 8 in R9C8 (step 7)
[Step 18a can be expressed more logically as
R19C4 = 9, R19C6 = 7 -> R1C4 cannot be 2 more than R1C6 -> R1C456 cannot be [513]
R19C5 = 3, R19C6 = 7 -> R1C6 cannot be 4 more than R1C5 -> R1C456 cannot be [315]
-> R1C456 cannot be [315/513] -> R1C456 = {126/234} …]
19. 15(4) cage in N3 (step 13) = {1257/1347} (cannot be {1248} because R1C8 only contains 3,5), no 8 -> R1C9 = 7, placed for D/ -> R9C1 = 8, clean-up: no 3 in R9C2 (step 6)
19a. R1C8 = {35} -> no 3 in R23C9
20. R9C456 = {136/145/235} (cannot be {127} which clashes with R9C28, ALS block), no 7, clean-up: no 2 in R1C4 (step 8)
20a. R1C456 (step 18a) = {126/234}
20b. 6 of {126} must be in R1C4 -> no 6 in R1C6, clean-up: no 1 in R9C6 (step 10)
21. 25(4) cage in N9 = {2689/3679/4678} (cannot be {3589} because R9C8 only contains 4,6, cannot be {4579} because 5,9 in R78C9 clashes with R456C9), no 5, 6 only in R789C9, locked for C9 and N9, clean-up: no 2 in R4C9 (step 14), no 4 in R4C1 (step 2), no 2 in R5C1 (step 3)
21a. 5 in C9 only in R45C9, locked for N6
22. Naked pair {35} in R5C19, locked for R5
22a. R5C234 (step 16) = {147/246}, no 8, 4 locked for R5
23. 23(4) cage in N1 = {1679/2489/2678/3479/3569} (cannot be {1589/2579/3578} because R1C1 only contains 4,6, cannot be {4568} which clashes with R456C1)
23a. R1C1 = {46} -> no 4,6 in R23C1
24. 45 rule on N3 3 innies R12C7 + R3C8 = 1 outie R4C6 + 18
24a. Max R12C7 + R3C8 = 23 -> max R4C6 = 5
25. 12(3) cage at R2C8 = {156/246/345}
25a. 3 of {345} must be in R2C8 + R3C7 (R2C8 + R3C7 cannot be {45} which clashes with 15(4) cage in N3), no 3 in R4C6
26. 12(3) cage at R6C6 (step 15) = {138/237}
26a. 1 of {138} must be in R7C7 + R8C8 (R7C7 + R8C8 cannot be {38} which clashes with 25(4) cage), no 1 in R6C6
27. 15(3) cage at R6C5 = {348/357/456}
27a. 7,8 of {348/357} must be in R78C5 (R78C5 cannot be {34/35} which clash with R9C456), no 7,8 in R6C5
28. 23(4) cage in N7 = {1589/2489/2678} (cannot be {4568} because R9C2 only contains 1,2,7,9)
28a. Killer triple 4,5,6 in R1C1, R56C1 and R78C1, locked for C1
29. R9C456 (step 20) = {136/145/235}
29a. R9C456 cannot be [514], here’s how
R9C456 = [514] => R1C4 = 4 (step 8), R9C9 = 6 => R1C1 = 4 clashes with R1C4
29b. -> R9C456 = {136/235}, no 4, 3 locked for R9 and N8, clean-up: no 3 in R1C6 (step 10)
30. R1C456 (step 18a) = {126/234}
30a. 3,6 only in R1C4 -> R1C4 = {36}, clean-up: no 5 in R9C4 (step 8)
30b. Naked pair {36} in R19C4, locked for C4
31. 6 in R1 only in R1C14, 6 in C4 only in R19C4, 6 on D\ only in R1C1 + R9C9 -> R9C49 must contain 6, locked for R9, clean-up: no 1 in R1C6 (step 10)
[I guess this is a sort of X-Wing using the diagonal, possibly XY-Wing, XYZ-Wing or some strangely named Fish.]
31a. 1 in N2 only in R1C5 + R23C4, CPE no 1 in R1C3
[This CPE has been there since step 9 but I’ve only just spotted it.]
32. 15(3) cage at R6C5 (step 27) = {348/357/456}
32a. 3 of {348} must be in R6C5, 4 of {456} must be in R78C5 (R78C5 cannot be {56} which clashes with R9C456), no 4 in R6C5
[Steps 29 and 31 gave me the idea how to continue.]
33. R456C1 (step 5) = {135/234}, R456C9 (step 14) = {359/458}
33a. R456C1 cannot be {234} and R456C9 cannot be {458}, here’s how
R456C1 = [234] => R456C9 = [458] (steps 2,3,4) => R1C1 = 6, R9C9 = 6 clash on D\
33b. -> R456C1 = [153], R456C9 = [539]
34. R4C5 = 3 (hidden single in R4), R23C5 = 15 = {78}, locked for C5 and N2
35. R9C6 = 3 (hidden single in C6), R9C4 = 6, R9C5 = 1 (step 29b) R1C4 = 3, R1C5 = 2, R1C6 = 4, R1C1 = 6, R9C9 = 4, R1C8 = 5, R9C8 = 7 (step 7)
36. R4C6 = 2, placed for D/, R2C8 + R3C7 = 10 = {46}, locked for N3 and D/
37. Naked pair {78} in R4C4 + R6C6, locked for N5 and D\
38. Naked pair {45} in R78C5, locked for C5 and N8 -> R6C5 = 6, R5C6 = 1, R5C4 = 4, R6C4 = 5, placed for D/
39. Naked pair {19} in R23C4, locked for C4, N2 and 21(4) cage at R1C3, no 1,9 in R12C3
39a. R23C4 = {19} = 10 -> R12C3 = 11 = [83], R7C3 = 1, R8C2 = 3, R12C7 = [98], R23C5 = [78], R1C2 = 1, R9C2 = 9 (step 6)
40. Naked pair {12} in R23C9, locked for C9 and N3 -> R3C8 = 3
41. Naked pair {25} in R2C2 + R3C3, locked for N1 and D\, R4C4 = 7 (cage sum)
42. Naked pair {28} in R78C4, locked for N8 and 22(4) cage at R7C4 -> R9C3 = 5
and the rest is naked singles.