The Belter/Inner dynamic is not a matter of left or right, it's a matter of colonialism. Yes there's some overlap in those concepts when it comes to workers' rights, but before the rights of workers can be recognised, they must first be recognised as citizens instead of subjects.
What the Belters want above all else is to be the authors of their own destiny. To not be dependent on the fickle good will of the Inner powers, nor subject to their capricious whims. The OPA is still a fragile alliance of factions with competing and even conflicting priorities. They've carved out a semblance of respect and unity, but with Earth and Mars at peace the Inners easily have the upper hand and any concessions could just as easily be taken away if the UNSG so wished.
All that said, the thing to understand above all else is that Marco doesn't actually care about the belt, he only cares about Marco. For at his core, he is a narcissist of the highest order. A sociopath. A gaslighter. A manipulator. Totally devoid of empathy, compassion, or a true vision of the future. The Belt under Marco's rule would not be the free and bountiful paradise he makes it out to be. It would be an authoritarian nightmare, with the Belters worked even harder because they'd think they're sacrificing for this great cause, but really, it's just another oppressor in a liberator's clothing. He'd do this not because he wants to cause suffering (again, he doesn't actually care one way or the other) but he has a driving need to control those around him, and by extension, *everyone* to satisfy his ego (which of course, it can never be so.)
There are a few very vital clues to his priorities and though process. The first is the conversation with Phillip after Naomi almost stabbed him. He's visibly emotionally shaken at the *thought* of personal injury. Inners can die by the millions and that's all well and good. His own people can die by the hundred and that's also all well and good so long as it serves his purpose. But the idea that he might be personally at risk *terrifies* him. Indeed he let the mask slip with Naomi by admitting she had a hold on him, which is the real reason he took Phillip. She frightens him, and he needs to control her to feel safe. Speaking of his personal safety, unless I missed something, I'm pretty sure his ship wasn't part of the ambush on the Rocinante, which to me means that after two failed assassination attempts (one sabotaged reactor and one IED'd ship) he's afraid to face Holden in combat. Despite possessing a superior vessel, firepower and numbers, he keeps out of the fray and sends lackeys to die in his stead.
The other major clue is his reaction to being informed that several of his ships were destroyed at the cost of only one UN ship. It's clearly not what he wanted, nor what he expected. Which also speaks to how far out of his depth he is given that he's sending advanced frigates manned by essentially pirate crews to engage seasoned professionals in conventional fleet action. Despite that he immediately pivots by making out that this is a worth exchange (it's not, nor is it sustainable given the disparity of numbers.)
He did pretty much the same thing when Earth got the Sentinel satellites pointed in the right direction and started shooting down his rocks. It's a pattern of behaviour. He never admits error. Nothing is unexpected. If something goes wrong, then then it's either someone else's failure, or it was really the plan for it to go down like that all the time. It's kinda like a reverse Xanatos Gambit where he perpetuates his image as a tactical genius by taking credit for the work of others (even able to convince those very people it was really his idea all along) and making out that no matter the eventuality, he always saw it coming (even when he didn't.)
Marco's biggest secret is that he really doesn't have a coherent strategy because he's a tactician. He can say that once Earth is bottled up and they take power that they'll create a better civilization...but he doesn't really know *how* to do that. He's way out of his depth and if he realises it at all, he's mostly banking on it just happening by itself. And if it doesn't...well of course this was always going to be way forward, for the good of The Belt...
