OP, it sounds like one of two things are happening with you. Or maybe a bit of both.
The first is imposter syndrome. This is from quotes such as this:
This is true, but I just didn't personally think I deserved what he suggested. It doesn't show disrespect, my grandmaster respects whatever it is that I want to do.
Imposter Syndrome is when you don't feel you actually deserve something you have. For example, you get a promotion at work, and then everything is new to you, so you don't feel you know what you're doing, and wonder if you were "promoted to your level of incompetence".
In martial arts, this is very common when people rank up and then feel they don't measure up. Maybe they got a blue belt in BJJ and all of a sudden everyone rolls harder, so it feels like they're worse. Or they got moved into the advanced class in Taekwondo where there's a ton of new moves that they struggle with, where previously they were doing well with the beginner moves. Or you're getting ready for your yellow belt test and you think you're not ready because you haven't mastered the moves, but the instructor merely expects you to have yellow-belt-level technique to get your yellow belt.
This is why people keep saying to trust your GM. Let me put it to you this way. I've assessed literally hundreds of students for eligibility for each belt test; thousands of total individual assessments if you add up all of the belts. I am not a grandmaster. I am not even a master. I assume your GM has done way more than I have for way longer than I have. Assessing students for belt tests is a skill he has honed, just as he has honed his martial arts skills. This is why we say to trust him.
The second is an ego-driven, rebellious sentiment, as can be seen in the following:
I do trust his judgement in most cases. I think double promotion isn't necessary and honestly shouldn't be a thing.
This is essentially saying that you have a better idea of the qualifications to be a green belt than your instructor does. You, who are not yet a green belt, are better at judging what a green belt is than the man who's probably graded thousands of green belt tests in his career.
Because these are mixed in with the imposter syndrome quotes above, it also colors our impression of those quotes as well. It sounds like you have a higher standard for each belt than your GM does. You are holding yourself to those standards instead of what he does. This is where the ego comes in. Your ego says that a green belt should be X, but your GM says a green belt should be Y. Instead of trying to figure out what Y is, you're criticizing your GM for not being X.
The rebellious sentiment gets worse here:
Not really. If I don't want it then I don't have to. So, not entirely up to him.
I don't know if there are many Taekwondo GMs that will tolerate "If I don't want to then I don't have to." Especially from a white belt. That's not how the culture works. There's leniency for bad attitudes at earlier belts, but in general, "I don't want to so I won't do it" is not the right answer.
My experience with this more has to do with young kids who only want to do the games and don't want to train, or older kids who want to do the opposite. Or discussions online with young adults who only want to do the kicks and skip the rest of class, because they think that's all Taekwondo is useful for.
If you want to do what you want, then start your own school. If you want to learn from your GM, then trust that he knows what he's doing. He's promoted a lot more people to yellow or green belt than you have. He probably has a good idea of how to do so. I personally have things that I dislike about my Taekwondo organization. However, I'm going to do things their way until I learn enough to separate from them.
Well, there's always gonna be people who have not experienced martial arts before; there is only one way to learn it. Start learning. So you shouldn't be biased against beginners.
There's a difference between beginners who have all the questions and beginners who have all the answers. We want people to come in and start learning. We don't want to gatekeep people coming in and asking questions. However, the problem is when beginners act like they are an authority in martial arts. It takes authority in a system to decide what the curriculum is or who is ready for promotion. That authority comes from experience, which you quite simply don't have. Which is why we're saying to trust someone who does (your GM).