My 21.R14? was a huge mistake. Somehow I looked only at the branch 22.t11 23.u12 24.u13 25.v14, and completely missed 23.u12 24.t12

Instead, 21.v14 22.s13 23.s11 would have been a quick win.

Despite this, I don't see a way for black to win after |23.n11.

32.f16 33.j15 34.k16 35.i17 would not have helped black.

What about 26.h12, with the threat of 28.e9?

E.g., 26.h12 27.f13 28.e9 29.d10 30.g8 31.e8 32.f4 33.e6 34.e2 35.g5 36.h5 37.i7 38.g7.

I think you are missing White's K7 threat, which lingers as late as

26.h12 27.f13 28.e9 29.d10 30.g8 31.e8 32.f4 33.e6 34.e2 35.g5 36.h5 37.k7 (following your line up to move 36),

but could be played earlier to good effect.

Yes, I absolutely missed that cool double threat. But now I must be missing something else, because I don't see how it actually saves white.

After 26.h12 27.f13 28.e9 29.d10 30.g8 31.e8 32.f4 33.e6 34.e2 35.g5 36.h5 37.k7

it seems black must play 38.m9. White can only take advantage (I guess?) through 39.i8, but 40.f10 seems to work against that:

if 41.h10, then 42.h11 43.d12 44.i7;
if 41.g7, then 42.d11

26.h12 27.f13 28.e9 29.d10 30.g8 31.e8 32.f4 33.e6 34.e2 35.g5 36.h5 37.k7 38.m9 39.i6, and then what?

Thank you.