For POSIX: I leave Bash as the system shell and then shim into Fish only for interactive terminals. This works surprisingly well, and any POSIX env initialisation will be inherited. I very rarely need to do something complicated enough in the REPL of the terminal and can start a subshell if needed.
Fish is nicer to script in by far, and you can keep those isolated with shebang lines and still run Bash scripts (with a proper shebang line). The only thing that’s tricky is `source` and equivalents, but I don’t think I’ve ever needed this in my main shell and not a throw-away sub shell.
I often write multi-line commands in my zsh shell, like while-loops. The nice thing is that I can readily put them in a script if needed.
I guess that somewhat breaks with fish: either you use bash -c '...' from the start, or you adopt the fish syntax, which means you need to convert again when you switch to a (bash) script.
I guess my workflow for this is more fragmented. Either I’m prototyping a script (and edit and test it directly) or just need throwaway loop (in which case fish is nicer).
I also don’t trust myself to not screw up anything more complex than running a command on Bash, without the guard rails of something like shellcheck!
I used to do it this way, but then having the mentally switch from the one to the other became too much of a hassle. Since I realized I only had basic needs, zsh with incremental history search and the like was good enough.
I don't care for mile-long prompts displaying everything under the sun, so zsh is plenty fast.
Fish is nicer to script in by far, and you can keep those isolated with shebang lines and still run Bash scripts (with a proper shebang line). The only thing that’s tricky is `source` and equivalents, but I don’t think I’ve ever needed this in my main shell and not a throw-away sub shell.