I agree with most of this, my biggest issue is how hard it is for me to recall any moderately complex shell syntax (or the slightly different Makefile syntax). LLMs largely solve that for me.
Here's a note from the Ubuntu recommendation: https://wiki.ubuntu.com/DashAsBinSh
The Bash manual also covers POSIX semantics, I just remember which is which. You're right that the Dash manual is a good place to check what is mostly POSIX (Dash isn't actually strictly POSIX). I would probably use Shellcheck with the correct shebang to double check what's compatible.
but if you don't, and the tldr command isn't available, then good freakin luck.