Your registrar should let you specify who is hosting the DNS records. Pick any host compatible with your client. Personally I use Namecheap and haven’t had to touch it in years.

how do I tell the machine to send piefed traffic to this subdomain and joplin traffic (for example) to another domain

You don’t send traffic to domains. You point all the domains to one host, and on that host, set up a reverse proxy like nginx, caddy, or traefik, and then configure HTTP routing rules. That proxy can run in docker. I use traefik and it does all the routing automatically once I add labels to my docker-compose file.

And only a 3.28 TB database? Oh, because it’s compressed. Includes comments too, though.

Only due to lack of capability, not desire.

Sales? No, their sales department is second to their legal department in hate.

I am once again recommending that you not expose any services to the internet except a VPN