I feel a need to revisit this thread now as I did go and buy the PC version at regular price, after hearing from some of the PC users here and other places. The PC version of 2K21, even on my somewhat average gaming PC, is night and day difference over the XBox One X version in frame rate performance! I unlock vsync on the PC version (which there's no option to do on the XBox One X version) and on probably 90% of the courses I've played, see 52 - 65 FPS in 2160p. The XBox One X is vsync locked to 30 FPS.
Why is the frame rate so important in this game? It's crucial for proper management of the tempo component of the swing. I'd been playing the XBox One X version in Pro-Am swing difficulty because of the tempo regulation problems I have with it. I average shooting 70 in Pro-Am on the XBox One X. Meanwhile, I find that I can play the PC version using Pro swing difficulty, and average shooting 67

I rarely get those inexplicable "off scale" tempo results on the PC, that I'm used to getting at least once per round on the XBox One X version. I think it's mostly down to tempo, at least in my case, and that tempo is so much easier manage when you're getting the feedback as to where the club is in the swing at rate that's twice as fast.
I reached my boiling point this week with the XBox One X version, and have decided to go exclusively with the PC version. I'm active in three societies on the XBox One and a lot of my friends play it, but I'm close to wallpapering my rec-room with a controller while playing it right now. If HB Studios (or Microsoft's backward compatibility team) does something to unlock the frame rate for the XBox Series X, as I'm hoping they'll do, I'll be back onboard with it... well, once I'm able to actually find a Series X that I can buy!