I recently received a Steam Deck and I love it!
I also planning to use it for development. On my PC I get used to Fish shell and usually use it instead of Bash. I can install it on Steam Deck and set it to the default shell, but after a system update it will be wiped. I can create a script and run it after each update, will I be able to login if my user points to /usr/bin/fish that doesn’t exist in the system?..
Simplest answer would be to launch Fish as the last step in your bash profile.
I imagine the SteamOS update process probably needs the default system shell to be bash, (if they’re changing it with each update) but that won’t stop you from getting the benefits of Fish during your login terminal sessions.
Source: I don’t like bash much, but have encountered other cases where changing the default shell was too invasive. It’s about an 80% solution, and the next 15% is covered well enough by invoking my preferred shell before invoking my script written for my preferred shell.