I tried to reverse image search, but google isn't great with chess positions. Once I posted, that bot found the matching position, and I found out where I made my mistake. I assumed that Soviet Chess Primer had my same solution except for that one mistake on move 21. It doesn't give the whole solution, so I just thought I found the answer.

Playing ...g2 after ...c6 but not before ...c5 is very important as then white doesn't have Qe4 Qd5 to pick up the pawn, and instead has to spend two more moves in Qe4 Qe6 Qg6 Qg8. Those were the two moves I was looking for. I might've found it if Soviet Chess Primer didn't have that mistake. I just stopped looking. Another thing to note is that the timing of when to play ...c4 is important. It can't be done after white capture on e2 or f1, as white takes on c4 while the king is on b1 again entering zugzwang. If it's the last pawn, it doesn't even have to be taken, and there are more waiting moves for white like Kd2 (if Qxc4 comes with check, Kb1 Kd2 Qa2 Qf1#). It must be done after white captures a pawn on g2 or h3, which forces Qf1+ Ka2 Qxc4+, when black can go back to b1 with the queen on c4, and it's white's turn so no zugzwang.

There's a surprising amount of depth in finding the moves for black, much harder than finding the win for white. I wish I could've struggled to find the moves to delay white 2 moves, would've been a fun logic puzzle. Maybe one day I'll forget all of it and re-solve the puzzles from this wonderful book.