I'm pretty bad at estimating anything, but those are 5 large glasses (albeit not fully filled), surely he's at least approaching a gallon, if not more?
For a while I split tunneled tailscale through an openvpn .conf file, but recently switched to using qbittorrent in docker with gluetun. Qbittorrent is realistically the only service that needs to be behind a vpn so it works out well
I'm sure any server oriented Linux distro will do fine. I use Debian.
I will note, I don't know if you're planning on having remote access (e.g. through tailscale or reverse proxy), but if you are, I found it quite a challenge to get proton to play nice with them
As the post says it's about the same quality as a Rick roll. Take that as you will