For what it's worth, unlike so many Asian works this does not indulge in the incest-kink while trying to hide it. They despise each other initially and there's no 'He loves me but I'm a naïeve girl who sees this as brotherly love' arc to begin with. Heck I think they've barely lived together before the game starts. Emotionally, this feels not like 'teehee maybe incest' but more like a grandchild of Pride & Prejudice, focusing on the way they both are horrible at understanding each other and expressing love and warmth. It feels more like the author included a 'not-brother' LI because 'that's how it's done' while having 0 interest in the 'forbidden love' angle of it, or what else inspires the trope in the far East.
And the age gap I think is in this case not so bad because the characters are otherwise on pretty even ground. Being expressly badly socialised by her mother, the MCs naivety is on the cynical side. She's not a overly trusting soft-hearted wide-eyed idealist ready to believe and follow everything he says. In fact the good ending requires her for most of the route to push back and avoid trying to please him every turn.
And in the smal exclusive community they're confined to, he doesn't hold more social or economic power over her. IRL I still would not trust this situation, but playing the game this felt like an 'age is a number' thing. She's neither overly childish or 'mature for her age' and he's not written as the big mature gentle protector.
The route is less about her thawing out, and certainly not about her being vulnerable or getting coddled, and more about him learning to have priorities and putting his loved ones first when push comes to shove.
(Haven't played Waltz' route yet, but so far I suspect it's much the same. He's the 'younger guy' LI, but unlike most he doesn't seem written to be the 'eager to prove he's grown up' guy, or the cute boyish boy. Hope I'm right on that.)
And unlike a lot of Asian works this is inspired by, there's no sexist 'just pinning you down reminding you to be wary of guys' stuff, no accidental kisses, no acting like women can't carry anything (Rod's route has an exception, but another girl in the scene could carry them just fine; in Karma's route you explicitly carry more than him), no making you feel small and weak and incompetent and never kind and forgiving enough.
I'm with you on the illusion of choices. Sadly Amnesia made the 'pick a path' formula very popular, but that means your choices only influence whether you get a good ending, with no indicator along the way you're on the right path. All that said, I've been really enjoying the game, and I hope I could inspire you to give it a try. After all, we can enjoy art and media while also acknowledging the problematic parts. Harmful tropes don't have power over us if we know what they are.