This is actually one thing I wouldn’t change at all for this exact reason. The reason the end of Beethoven’s Op. 131 is so powerful is precisely because the C-sharp major is so “unconvincing” after the intense lean towards F-sharp minor throughout the coda; it creates a sense of infinite space in the ending, leaving it as both extremely final and extremely open, a question mark that at the same time feels like the only possible way the piece could have ended. Beethoven pulls a similar trick at the end of the Heiliger Dankgesang where it’s impossible for the final F major chords to not feel like the subdominant of C major, given the Lydian mode, and yet they are the only ending that could have possibly worked; introducing a B-flat into the texture at the last minute would have been much too disruptive.
That’s just my opinion obviously, I am just one person, your thoughts and opinions here are of course much more important. But my view overall is that I preferred the C major ending in your piece with the sense of ambivalence as to whether this was a true “resolution” or not. I like uncertainty in endings in general, though.
Thank you for your thoughts on this!
I think I've settled on the version below. I don't have a strong need to make the ending key perfectly convincing, but in the original version from months ago, F minor had been established so strongly before - not just in the coda, but in the final few pages of the last fugal section. It was so strong that it was not ambiguous at all IMO, but as in Op. 132, impossible to hear the final chord as the tonic.
But in Op. 132 the effect comes, as you say, from the whole movement being in the Lydian mode, so that's different. Op. 131 plays a different trick and I'm realizing, now that you mention it, that it's the exact effect I'm after here, a leaning toward the subdominant minor but only a leaning, producing some ambiguity but still with a strong sense of tonic in the final chord.
I'm keeping my changes to the ending of the fugue for other reasons, especially that I really like the crossing and re-crossing of the violin lines at the climax and the climbing into the stratosphere in the aftermath. But the coda had to return all the way to Earth and carry the full weight of the rest of the piece. The only change I'm making to the Coda (other than some harmony and voice-leading in one spot) is to begin in C minor, then four more overlapping entries instead of three, descending into the lower registers. The final cadence is supposed to have an element of Stoic triumph, which would be missing if the whole section were just transposed up to C minor.
Audio file:
https://drive.google.com/file/d/1oyOHzK7ephVdpICzk-r6ZrmsWQEUzl6O/view?usp=sharingScore:
https://drive.google.com/file/d/12YkAPtdsjSW4vxr48sjBfv05gt3SpKFX/view?usp=sharing