Linux

Bash. I used ZSH for a while too, but without Oh My Zsh, installing only a handful of plugins manually (or I used a simple plugin manager, honestly forgot if). It was nice, but I had ton of stuff that was not really necessary. And with the next OS install I decided to try a Bash only system. And you know what, it feels so much better and I appreciate simplicity a bit more than before.

But I think you won't go back. Fish is nice first, but it's too different from Bash and Zsh. I had constantly think about the differences and most of my scripts are Bash anyway. So I did not want to have a scripting language that is different from interactive shell. A language like Fish is too similar, while being different. This messes with my brain, the language itself seems to be fine.

What's left? PowerShell... nah, just joking. The problem is, most are different languages and not like Zsh and Bash at all. You listed Nushell, there is also a Python like shell language Xonsh. There is also a C like one Csh But to be honest, if you want a POSIX compliant one, then you don't have much to choose from. Either start your ZSH setup from scratch, with the knowledge you have now, or go back to Bash. That's what I did and kept using it since.