This would be the ideal solution for everyone involved. Give them autonomous rule at first but with military, economic, political etc. oversight.
Yemen has proven, time and time again, that it has no future as a strong nation state. There is a reason why the nation state (Yemen - not talking about ancient Yemen or ancient South Arabia as a whole - which was rarely ever fully unified as well for that matter) of Yemen was emerged in 1990.
The local Hadhramis would have a 1000 times better life (in every way) if they joined KSA. They are arguably closer to Najd as well than North Yemen.
If the leadership is bold in this regard, they need to make the Saudi Arabian presence in Yemen permanent and after a while propose a referendum if the Houthi terrorist cult is still around in North Yemen. Even if they are not.
90% + of all locals would vote for joining KSA without even thinking twice.
I would argue that most Yemenis would.
This would solve a lot of things as well.
After that work (long-term) to annex/unify with the remaining GCC.
If/when current regimes are gone - leadership across Arabia should work towards full unification once again. IMO, just a question of time.