The second bit is that it’s much easier to stay frozen in time than bring a new-found old Mac back to life decades later ;-) If you had been using those Minis in the late 2000s, you would have already had installed lots of software, which would have kept working as-is (you wouldn’t have been able to upgrade to new versions and get new features), but you wouldn’t have had to chase down old versions of software that would work on the old Macs/OS versions. (Thankfully, many indie Mac devs like Red Sweater and Bare Bones and Flying Meat are good about keeping those versions available, if unsupported, for people with old Macs.) You’ve taken on a minor challenge, but it’s definitely do-able with defined usage goals. Feel free to holler if you need more advice once tax season is over and you dive in :-)
* I haven’t found lots of sites that require TLS 1.2, but I’ve run into enough of them to find it annoying on my very old MacBook Pro.