You want a prebuilt NUC. That is pretty much it. A Pi generally doesn’t cut it, plus by the time you get all the accessories to make it usable as a server, you are at the price of a much more powerful small PC.
I run a ryzen 3600 with 32GB RAM in a Node 304 and it is very quiet with 12TB red pro helium drives. 4 HDD capable with a GPU and 6 without. However, it still draws 20-30W idle or 50W with an Arc A380 installed.