I would go with 16GB for the kinds of things you listed.
If you don’t need a 3.5in drive then I would go even smaller to a mini form factor. It will definitely save you money on electricity. I think it’s the main choice here.
I would go for refurb, business line SFF machines. Something like ThinkCentre or Optiplex. Specific form factor based on drive needs but the smaller you go the more power efficient. I have one on the bigger side (internal psu) that runs about 12W idle.
Just double check that it can handle hardware transcoding. Should fit right in your budget!
I use Immich for sharing. Get some accounts set up for closest family so you can easily add them to albums. For others you can just share a link to each album, password protected or simply unlisted.
Personally, I run my internet accessible apps on my Hetzner VM behind a reverse proxy, whereas things like home automation, DNS and Octoprint I prefer to serve on my local network.
I guess if the serious thing you’re doing is a CRUD app?
Tell that to YouTube and Spotify. Or astronauts on the ISS. Or anyone doing science really…
Machine learning and CV? Geospatial? Math in general?