A script, by its very nature, is synchronous. You run, you get a result. If you want a service, don't use a script.
This is pedantic to the point of being conversationally hostile.
That’s a completely arbitrary limitation which should not exist. When my scripts contain independent non-instant subprocesses, I run them in parallel. I’m also using js/ts to write all my scripts, because why wouldn’t I use a proper programming language instead of that “bash” thing that can’t even handle its single datatype well and resorts to all sorts of gibberish to perform trivial operations on it.