As others just said, take the challenge of repairing or correctly identify what's broken. It should be fun, and part of the diy ethos.
If, you are still going for a new piece for the upgrades...
Depends very much on your scope. Heck, there's even aesthetics. Do you prefer AMD or Intel? Don't you feel like having an ARM and take the challenge of compatible images? Would you need to check the transcoding compatibility for Jellyfin? Are you going fangless? Is one 2.5gpbs eth port enough? Etc.
That said, check the reviews from https://www.servethehome.com/ they have articles with benchmarks and videos for all the latest nucs and other hardware's form factor servers.
Be aware that Intel discontinued the NUCs iirc.. but Asus or some vendor might have picked up to grab that market segment.
Good luck!