The spare one is a 5600X which was previously in the same motherboard before I switched to a 5800X.
I am considering getting a B550 motherboard once I have the funds, but the number of SATA ports is kinda low. My current one is X570 with 6 ports.
I am not planning on RAID for now, as I don't really plan on ever having enough for redundancy, but I definitely want to be able to use the system for any and every task I can program to control via a local SSH. So definitely going with a full distro, like Debian or sth, and not a NAS specific thingy.
I can even make use of my old Radeon 4650 for a display while setting it up.