So I was in your exact position - I really wanted the i9, especially since I'd be upgrading from an i9 (9900K) and hated the thought of "downgrading," even if I knew I wasn't.
So I bought both and decided I'd test both. If the performance of the i9 was clear, then I'd stick with it. I'm going all out elsewhere (3080 Ti, 64GB, Gen 4 2TB PCIe, etc).
Between Halo Infinite, FFXIV, Doom Eternal, and Cyberpunk 2077, I was seeing ABOUT 2-3 fps higher on the 12900K vs. the 12700K at 3840x1600. Cyberpunk saw the biggest difference - about 5fps at all times.
Here's the deal, though - on my Fractal Torrent with Noctua DH-15, I was hitting 10c+ higher in pretty much every single game. From 55-65c on average, to 75c-80c across the board. In Cinebench, I went from maxing out at 74c on the i7 to hitting 94c on the i9. Sure, the i9 got a better score - but for those temps (in the winter no less) didn't seem worth it at all.
I returned the i9.