My home lab was similar not too long ago, it’s a slippery slope once you start upgrading!
What sort of bandwdith do you think you may need for your racing game? What kind of connection are you on now if I may ask?
That’s great, self hosting is awesome! I have a bit of a hard time liking Docker, I like the ease of installing well groomed containers through it (and use it for some of my services), but it can be a nightmare to get under the hood for things like networking.
My go-to is running everything in separate VirtualBox VMs, they are really easy to run and back up, everything is nicely isolated, I can control resources well, etc. Where possible I’ll install services without using Docker inside of these VMs.
If you want to keep an eye out for old server hardware, SuperMicro X10 motherboards and Xeon E5 processors are a fantastic generation of hardware that is currently being decomissioned, and they run on DDR4 which is also starting to be cheap now. Since you’ll be running this stuff at home, it also matters how loud it is. My advice is to stay away from HP/Dell/IBM servers (even though some of them are dirt cheap) as they all use propietary/custom motherboard layouts that won’t fit inside of a standard ATX case. SuperMicro doesn’t (with the exception of some specialist or 1RU boards), so you can transplant them inside of any case large enough to take an E-ATX board. You can build a whisper quiet, powerful server!
I had NextCloud running for a while and I think I tried ownCloud, it was alright but I was mainly using it for back-ups and it wasn’t great for it (a bit slow, a bit temperamental). I now use Syncthing to backup important files between machines. It’s secure and sends things at line rate, but if you’re looking for more than just file back-ups then something like NextCloud/ownCloud may still be a better choice.