Poster here! ble.sh gives syntax highlighting for bash just like fish and zsh. I personally don't like zsh because it seems over engineered and fish have some design problems (I don't think they have yet fixed them).
The feature-rich alternatives you mentioned are exactly what I was thinking about (and using daily!).
There are a few things about zsh that "feel better" than bash (especially arrays), but given the ubiquity of bash, if I can get the same general quality-of-life upgrades that I experience from the zsh community add-ons, I may need to give ble.sh a go.
Edit: I gave ble.sh a try, and it is super-cool-magic, but does feel slow with quite noticeable input lag.
The feature-rich alternatives you mentioned are exactly what I was thinking about (and using daily!).