I often see people complain about this but I seriously don't understand. The shebang at the top of a bash script will point to bash no matter what your shell is.
At various jobs I've often seen bash scripts to set up your environment (mostly environment variables). Running a bash script with shebang will not preserve the changes to the environment variables, will it? At least that was my experience.
I use xonsh and fortunately it has source-bash for stuff like that.
I've never needed it, but there is bass for this use-case: https://github.com/edc/bass