OK, I've given it a good whole year. I'm calling it: zsh's supposed benefits don't justify the move from bash. I'm gonna get the latest bash and go back to using that.
I resisted until I got a new Mac and then I thought I’d give it a fair go.
Conclusion is, the little paper cut differences just don’t pay for themselves.
(Analogy is “I’ll just do that one endpoint in Y, when rest of app is in X”.)