Well, on a scale of 1-to-10 difficulty, the main character's situation is 11/10 completely and utterly hopeless long-term for freedom.
The succubus goddess is all-powerful in her pocket dimension. He's already been infected with her mark. Her most zealous and emotionally-bonded servant is literally hovering over his shoulder ensuring compliance with physical force at all times. What few powers the main character has derive from his overlord directly and could probably be taken away on a whim. And he'd also need a way to overcome her ability to influence and reprogram emotions hypnotically. It seems to even be implied that if he rebels, a hex will automatically activate (maybe?) and curse him for eternity, violating the contract he signed.
Compounding everything, there's nothing left of his home to return to. The central government of the kingdom he's been isekai'd to literally worships the person who enslaved him as a deity and as a matter of national identity--the "succubus kingdom" as in the title of this project I'd assume. By extension any followers he recruits there will likely adore her too.
This MC is basically stuck here for the ride and is probably limited to trying to make best and enjoy his situation--plenty of action there. How you'd write a rebellion route out of this, I'd have no clue. At best he could strike a deal with one of the other four supreme beings vying for power, but he'd basically be trading one succubus mistress for another with a new binding contract.