Thinking about this a bit more, it seems that the reason there isn't a good way to sell licenses to software libraries generally is license enforcement. Unity and Unreal have a licensing system built-in that they enforce against gamedevs. Normal server software has no such thing.

That means the only threat you have as a producer of code (once the code is handed over) is the threat of withdrawing service. That means the only ways to sell licenses are:

* Build your own licensing service (or offer SaaS)

* Sell the code for a high price upfront

* Sell service contracts