Have a previous gen 4K, and have not encountered any issues with Jellyfin on streaming. There’s a spectacularly annoying bug that you lose your config if the atv is full to capacity - and with kids in the house it means frequent logins are required. The iOS client also seems to lag on features and updates compared to the other clients, but other than that niggle it’s been great.

Depends if they are in charge I guess.

There’s a few companies selling a very plasticky mini-pc that also has 2x 3.5" slots. Trialling it now for a homebuilt Nas, so far impressed. Worst problem is that the bigger drives can be noisy.

Aoostar in the US I think.