Shortcut's Run Shell Script won't let me change Shell

You override the Run Shell Script Shell selection by doing the following on its first line:

#!/opt/local/bin/bash

and the rest of your Bash code. This works for me using the current Bash 5.2.32.

Here is how you get your Bash 5.2.32 to appear in the Shortcuts Run Shell Script Shell menu. You append that Bash path to the /etc/shells file. It is a read-only file and I was able to add my Bash 5.2.32 path to the end of that file using Sublime Text 4:

sudo subl /etc/shells

and then, I was able to use the 5.2.32 shell in the Shortcuts Run Shell Script:

However, I was never able to get past Run Shell Script errors when using this as a Quick Action and right-clicking on an image. If I set a variable to a hard-coded image file path without a for-loop and then just click the ▷ Run symbol, I get the following:

I tried everything for three hours and could not get the Quick Action with a for-loop working. Always has the same error notification. It would probably work fine if you were using the Zsh shell ;-).

Did you grant this version of bash Full Disk Access ?

I have never used MacPorts, only homebrew, and I left Bash long ago for Zsh. I will now remove Bash 5.2.32 and its /etc/shells reference as this was for testing purposes.