When people mention `bash` it's immediately a code smell. In order to write portable shell scripts, it must be only POSIX `sh`. If the need ever arises for a more complex data structure, typically I jump into AWK since it's also POSIX compliant.
Not every shell script has to be that portable. And even then, "POSIX compatible" shells vary in their "POSIXness". Sometimes using arrays is the right thing to do, or having a reliable indicator of the source script location, so you code to Bash 3. Sometimes associative arrays or something else is better, and your platform is known, so you code to Bash 4 or 5. Maybe all your users are Zsh users so you use that. So you can start by writing POSIX compatible scripts, but there's no sense in tying your hands if you don't need to.
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.
I've never been in a situation where I had to care. Bash is everywhere, it's a de facto standard of its own. Even a lot of buildroot and Alpine based Linux deployments, which don't come with bash by default, usually have bash added to them.
It doesn't come with bash pre-installed, but installing it is as simple as "pkg install bash", and that is the option that everyone working with FreeBSD will choose when they encounter a shell script that only works with bash. My point is that bash is already so ubiquitous that almost no one is going to care about an extra few megabytes for the bash executable in their OS image.
If you're installing packages anyway, then why not just write in Python, Go, or any other languages which have relatively less footguns and have more features? I'm sure at least some of those packages are also available on FreeBSD.
Here's a note from the Ubuntu recommendation: https://wiki.ubuntu.com/DashAsBinSh