IMHO fix whatever you can, donate it all locally (HackerSpace, RepairCafe, Linux non-profit, etc) as there are quite a few people dedicated to refurbishing computers for schools, people who need a computer to find work, etc.

Then for the tinkering aspect, keep one, that's enough.

Honestly even 1 isn't really required. Pretty much everything listed here can be done more efficiently without an actual physical computer :

• your current computer can be a server, just turn off the screen or even accept (which I'd argue is a fair assumption) that at night it will be off. If you want external access put WireGuard or another VPN on it.
• Want to test distributions or anything else? QEMU or containers, no need for actual hardware