Hi infu,
I am a researcher on the boundary node team. Custom domains are on our roadmap and we will get to it rather sooner than later. For more information have a look at our roadmap update.
We already have a good idea of how to add support for custom domains and it matches many of the points you brought up.
At a high-level, you will have to direct traffic to the boundary nodes by using a CNAME in your DNS entry. You also need to specify to which canister the domain should be mapped. Then, a couple of things need to happen in the boundary node such that they can terminate TLS for the custom domain.