I don’t think Tailscale counts as a reverse proxy, but it does support HTTP/3 and QUIC so that suggestion may not apply here. Still, it might be worth double-checking to see if they are disabled. Also, have you tried disabling Tailscale altogether and connecting directly?
For those on the fence about Borg Backup because it’s a command line app, FYI there’s a great frontend GUI for it called Vorta (yeah, in line with the Trek theme lol) that works really well. I don’t see it mentioned often, thought I’d pass that along. Might want to avoid the Flatpak version if you need to back up stuff outside your /home dir.