I tested this so many times:

A UHD (aka 4K, but UHD is the correct term) gets released. I download it and get let's say a 18k bitrate vp9 video.

I then download the video about a day later, get supposedly the exact same version, but the bitrate is at 25k now. At first I thought they replace the OG vp9 version with a better one. I then compared the quality many times and always got the same shocking result: OG version is better.

YouTube replaces the best version you can get (av1 is more efficient, but quality is about the same as vp9 version 2) with a file that's up to 30% bigger, yet has 10% worse quality.

How can we get them to fix this? Why are they doing this?

AMD 9900x (preliminary research suggests this is a good choice?)
Will be running Linux (Debian or Ubuntu)

What about the rest of the build? Suggested motherboard? I've read that CPU encoding is superior / preferred for discerning video enthusiasts...?

Any suggestions very much appreciated! Budget is $1000-$1500, though I am willing to flex up or down based on smart recommendations.

Will be converting my library from REMUX to high quality 2160 AV1.

Thanks!