1) You're missing the point. Overseer can do this, yes. AI model can work with ANY REST API.
2) How does Overseerr help? I've never really understood it if I could give my family access to Overseerr over a VPN I could just give them sonarr / radarr directly.
Expose Overseerr over HTTPS either from your home network or on a free VPS which has the tunnel into the home network where Radarr and Sonarr live. This is simpler for users to go to a website instead of having to also use a VPN.
This of course assumes you already have Plex or Jellyfin exposed to the public internet without a VPN.
If you're already forcing users to use a VPN for Plex/Jellyfin then hosting Overseerr locally and making it accessible over the VPN should be trivial.