Any possibility to try optimize the CPU utilization of the seated passenger to not using the main thread but distribute among the unused threads?
Seated Passengers are NOT handled by GSX, only walking ones.
Seated Passengers are created by modifying the airplane exterior model (or the attached_object.cfg for 2024 native airplanes), with an added visibility condition so you can optimize them by opening/closing the cockpit door and controls their appearance on every seat so, other than setting those variables, they are completely outside GSX control, something you could have verified very easily: you can manually quit the Couatl engine, but Seated Passengers would still be there, precisely because they are not handled by GSX.
So, if you are referring to MSFS 2024 and the issue that with SU3 even invisible passengers have an effect on fps, it's an issue that started with SU3, and has been FIXED in the SU4 Beta, because only Asobo could fix it.