+1 for Hetzner -- it's an amazing webhost and traffic probably wouldn't even be an issue there. A Hetzner cloud instance with one of their volumes would be great -- has support for Infrastructure-as-code (via terraform, pulumi), so write that once and you're good (you can make it open source if you want and others could help, since all the credential stuff is private obviously).
$300/month is a BEEFY instance that you will mostly waste -- a smaller instance that you can possibly scale horizontally (if you really need it, 99% of the time you won't, just add more bandwidth as you need extra).
Should be mostly set & forget though initial setup might take some time, it's even faster if you use discourse's official docker image -- then the steps are:
- provision a cloud instance w/ a nice big volume
- do basic hardening (ufw, fail2ban)
- install docker
- install caddy (could be a docker container)
- point caddy at docker container running discourse (caddy will handle your HTTPS cert retrieval)
- take backups and send them to a Hetzner storagebox w/ a cron job