I don’t want to get in the way of your argument re. Usenet, but spinning hard drives will last longer if they stay on. Starting and stopping the spindle motor will impart the greatest wear. As long as you have the thermals managed, a spinning disk is a happy disk.
Just a couple thoughts (I have a mix of 2.5Gb and 10Gb):
Mikrotik switches are a nice alternative to Unifi. Much less lipstick on the UI but reliable and fairly priced.
If possible, you’ll probably want to use your own router rather than the all-in-one provided by the ISP. In my case, the router provided to me (Eero brand) did not even have a port fast enough for my service, and would have been an instant bottleneck.
Options for 10Gb-capable PCIe adapters (what you might put in your server or desktops) are more limited (at least they were when I transitioned a couple of years ago). Intel-based network adapters seem to require less effort to get working (driver-wise) vs. some of the other 10Gb / SFP+ capable adapters.
Finally, you are correct: nobody needs an 8Gb internet connection. Aside from well-seeded torrent file transfers, you will never reach that limit (and probably still never). And, you’ll need an adequate storage backend to write that fast.