Looks really polished and I love the minimalist design. One thing I've been wondering that a lot of web platforms offer: how do you allow users to bring their custom domain with TLS? What's the tech behind that and how does the process work?
Things get trickier if you want to handle this well for globally distributed servers, since you'll need to have a cluster of reverse proxies near or colocated to your app servers. That needs an anycast IP address to handle A records for apex domains, and usually you want them coordinating to share certs, cache, etc. efficiently. In that situation I'd recommend reaching for a paid service, since there can be a lot to build and maintain.
Source: I built approximated.app, which is a service that does all of that for you.