…which does not support Firefox sync to sync its (container)settings

I don't see any reason why it couldn't. It's just a pull request away!

Why do we need this?

I'd have every tld on it's own profile and allow creating groups that merge profiles if needed.

Have tried that - but the patterns there aren't case insensitive (and no way I could find to make them so); the one that I use is a fork of containerise - search for bifulsushi on bitbucket