I am currently working on a FiveM server project, I am gonna get paid mostly in equity (getting a % of the revenue on every microtransaction etc), I am wondering though, how do I make that legally-binding? and how do I still protect my share in case tebex somehow gets changed for another monetization platform?
I think nobody gonna help you with a “legal-binding” that you are supposed to be paid by someone else. The only way to resolve this is through a contract with your “supervisor”. 
As for Tebex, it can’t be changed because according to the terms and conditions, the only possible platform available for having microtransactions on servers is Tebex and you simply can’t change it, because it wouldn’t be legal according to terms.
You need to write a legally-binding contract. If it is a big project, it may very well be a good idea to hire some cheap lawyer to just get everything set in stone. Gotta get the names and ID’s of the project “owners”
Short answer - get a lawyer and have a contract written with all of the other people.