Glad we could be of help :)

Using it as an on-demand machine sounds like the perfect middle-ground! My jellyfin-server is also not running 24/7. Sometimes I think I could as well host jellyfin on the same machine that I'm watching/listening stuff on, as I'm the only one using it. But I started similar as you - I had a machine laying around, and wanted to do something with it.

When it comes to upgrading your machine, which I guess is still years in the future, as you asked for 0$ tips, I'd suggest first seeing how much "compute power" you actually need. People tend to oversize their machines (that also is a problem in data centers [or at least used to be, not sure about AI data centers]), so before you upgrade your machine, check how high its utilization is.

The hill I'm willing to die on is saying that people have too much RAM, and underestimate swap.

As for beginner-friendly Linux distributions, I'm not sure if I can help there, I feel like I'm still somewhat winging it. But there seem to be articles and wikis for everything, or someone somewhere had the same problem and others with more knowledge helped out.

In general, and I'm not sure if that's blasphemy, I'd say that most Linux distributions are interchangeable. They mostly only differ in the packaging system, the default desktop environment, and the preinstalled software. At most you get a different init system. (Please someone correct me if I'm wrong :D)

I sporadically tried Ubuntu ages ago, then "seriously" started with Manjaro (should have gone for Arch, in the long run Manjaro seems to be just causing problems), and then switched to Void Linux. Arch has a nice wiki that is useful independent of what distro you run. Void Linux also has a nice handbook.

I like the minimalistic approach of Void Linux, and I think it was recommended for old computers somewhere (any distro coming with xfce by default probably is recommended for that, but Void Linux really comes with basically no bloat). It requires some tinkering, as the base install is, well, pretty basic, but it's doable. I can play games on my desktop PC and have jellyfin running on it in a docker container on my old (now headless 💀) laptop. My recommendation for it would depend on how well you are versed in using Linux in general.

Otherwise, as I said, Linux distros are pretty interchangeable. Point at one that looks nice and shoot. If you don't like it, just try another one, reinstalling (at least when you don't have a lot of data stored on it yet) is not really a big deal.