Same. I think you just use transactional-update instead of zypper when you're ready to install something permanently. I don't know what the package selection looks like though.

But anything that's going to need frequent updates should probably be installed outside of transactional-update. For compilers, I generally use some kind of compiler version manager (e.g. rustup for Rust), which would be outside of the immutable base.

There's a KDE version as well called Kalpa.

The main benefit is that the base system is immutable, which should make it much more reliable. So to get new software, instead of doing a zypper install ..., you'd leave the base system alone and only install packaged apps (e.g. flatpaks). That way you don't get conflicts between package versions and upgrades can be predictable.

I don't have direct experience with it, so I don't know how well that works in practice. But I definitely like the idea of using it for my NAS, which doesn't need a lot of updates. It currently runs Leap, but MicroOS should be a relatively straight-forward switch.

I haven't used it, but this post implies that it should work:

edit /etc/sysconfig/displaymanager and put your username in DISPLAYMANAGER_AUTOLOGIN="username"