I would replace the fans, to start with. The 550 could take extra RAM (see https://taoofmac.com/space/com/cobalt/raq550 https://taoofmac.com/space/com/cobalt/ra

I would replace the fans, to start with. The 550 could take extra RAM (see https://taoofmac.com/space/com/cobalt/raq550 https://taoofmac.com/space/com/cobalt/raq550 and https://taoofmac.com/space/com/cobalt/faq https://taoofmac.com/space/com/cobalt/faq), and it was possible to get a decent(ish) kernel on it. Alternatively, consider getting a cheap Celeron N5105 mini PC (or two - two should fit in the case) and adapting the PSU for it… Might be a fun project if you can do it non-destructively.