Linux

Debian based systems for systems you need more stable. Generally not a big concern for me unless you are setting up servers or really depend on specific software.

I wouldn't recommended Ubuntu, I would recommend something like mint if you're going that route. But Ubuntu is a safe feeling first choice and i get why people chose it. If you google a problem and just add the word Ubuntu generally speaking the google result or even googles ai summary has got you covered pretty good.

If you are more into gaming I would suggest something like bazzite, cachyos. Often they handle drivers for you super easily and comes with things like game scope too. Often you want more bleeding edge operating systems for gaming/newer hardware as updates seem to happen more frequently and with more noticeable improvements in the past 5 years.

If you just need a basic computer the choice doesn't really matter all that much its just kind of what looks good to you. You said its just for older computers so something like mint would be a fine choice in this scenario.

Generally speaking you are either picking a arch based system or a Debian based system. And all the distros are kind of the same, they just give you preinstalled software, technically you can pick any and get the features you wanted yourself.

Hopefully I didn't confuse you more or give you too much decision paralysis.