I'm not convinced about having shell scripts end with ".sh" as you may be writing a simple command style script and shouldn't have to know or worry about what language it's using.

I'm a fan of using BASH3 boilerplate: https://bash3boilerplate.sh/

It's standalone, so you just start a script using it as a template and delete bits that you don't want. To my mind, the best feature is having consistent logging functions, so you're encouraged to put in lots of debug commands to output variable contents and when you change LOG_LEVEL, all the extraneous info doesn't get shown so there's no need to remove debug statements at all.

The other advantage is the option parsing, although I don't like the way that options have to have a short option (e.g. -a) - I'd prefer to just use long options.