I think lemmit.online is already doing something like this and that whole instance is designed for it. They even have a community for suggestions on which subreddits to do. I would start there to avoid having to completely reinvent the wheel (unless you are into building things then by all means)
Either you need to expose those two services to the Internet or use a VPN. You mention vpns are inconvenient but have you checked out tailscale? The free account does your needs and it’s by far the easiest one I have set up. You can also look into a cloudflare tunnel but you would need a domain name for that