And it's back. Came back several hours ago and has been chugging away ever since. Guess I will be reinstalling Mac OS from the grounds up.

It looks like this did it - https://developer.apple.com/forums/thread/79354 I just went to Settings -- Apple ID - and unchecked iCloud Drive - waited for that to process - then rechecked. Took 4 hours to download my 40+GB of iCloud data but after several restarts and using my MacBook like normal - no thumbnails agent. Also I'm noticing a vast improvement in OS speed having done this... I remember my Activity Monitor always being in the 20-40% after upgrading to Big Sur. I'll report back if I see ThumbnailsAgent again but so far so good.

Big Sur 11.2.3 - still an issue. I wake up to com.apple.quicklook.ThumbnailsAgent consuming 142.9% CPU - 170+% CPU. diskarbitrationd is always the next high CPU consumption app. This is slowing down the OS and making working throughout the day noticeably laggier than my older MBP. Killing the ThumbnailsAgent only works temporarily as it always comes back. No amount of letting it sit there consuming high CPU resolves it either. Restarting Finder does not work. There is no such command qlmanage -r disablecache - The -r cache doesn't do anything. I'm getting ready to just wipe this MBP and start fresh because my work provided older MBP doesn't have this problem and it runs a lot faster than my 2020 MBP. What a shame.