You are right about the contract , but it doesn't change the fact that the FH together hold most off jahad empire influence and power if they decide he is not king Zahard likely won't remain king for long .
There is still a difference between acting like a subordinate and being one .
There is also a difference between being the king and being a irregular king .
We know Arie is Zahard servant and decided it , what is the nature off they relationship ? We know it is complicated and it isn't a relationship boss servant for the most part .
Would a FH move if zahard want it ? Well we don't know . We know Rei and Arie are loyal but it doesn't seem that easy .
For example for people who have any idea how power work in autoritarian regime , there is a king or a leader but that king always have ally that he can't afford to piss off , those who hold key position in a country , for example the head off the military who can control the army and send it to kill the king if he is pissed etc etc .
All off that is to say , that zahard power is sustained a lot by those FH , not only that like most regime the king doesn't have enough power to overpower other family head , so he has to act right if he doesn't want to be replaced .
The thing is that the argument don't hold one , because Gustang doesn't need to be stronger than zahard , if he has enough irregular with him , zahard will be in danger .
Not only that , it seems there are multiple time where it said that zahard consider FH also has friend .
