I’m curious to know, are you actively developing all 5+2 codebases, or could you theoretically run half of them in docker and another half (the ones you’re actively changing) on the host?

That's something I've seen many times said by many people but I don't understand. Your code can be mounted from the host into the container. There's no need to rebuild images with each change. Inside such "development" container some form of watcher/auto-rebuild can run and recompile & restart your program after changes are made in code. Isn't it standard practice?

What about isolation? Of runtime, ports, dependencies?

I think a lot of people here can't imagine how this is possible. Docker is literally just the same thing as what's running on the server except in a chroot environment. And it gives you a ton of benefits like isolation, portability, infrastructure as code, a layer of security, etc.