In your situation, where you want to restrict access to non-paying users, personally, I'd never use a library.
If someone can view your code, then they can duplicate it, . . . put it into their own Apps Script file, and run it.
That would be illegal for someone to do if your code is not licensed for free public use,
but how are you going to track them down and prosecute them?
It's never going to happen.
Would anyone ever steal your code?
I don't know, maybe, maybe not.
If the Apps Script team would provide a way to lock down library code,
then (in my opinion) it would open up a powerful and popular capability.
If it were me, I'd make it a priority.