With certbot there’s probably a plugin to do it automatically, but if you just want to get something working right now you can run the following to manually run a dns challenge against your chosen domain names and get a cert for any specified. This will expire in ~3 months and you’ll need to do it again, so I’d recommend throwing it in a cron job and finding the applicable certbot-dns-dnsprovider plugin that will make it run without your input. Once you have it working you can extract the certs from /etc/letsencrypt/live on most systems. Just be aware that the files there are going to be symlinks so you’ll want to copy them before tarballing them to move other machines.

certbot --preferred-challenges dns --manual certonly -d *.mydomain.tld -d mydomain.tld -d *.local.mydomain.tld

Yeah I can’t say I’ve used it myself but it seems pretty straightforward and very in line with SteamOS philosophies.

The steam deck uses KDE Plasma 5 as its desktop environment, so anything that uses that should feel very similar. I recommend bazzite if familiarity is something that would appeal to you.

Nah sounds like a waste of time and resources to make a game built on their multiplatform engine and then lock it down to one environment.