OP, I agree with a lot of the other comments and think you should write him a letter. I think that your Dad is trying to avoid thinking about how shitty he has been treating you, and I don't think having a conversation where he can input will be particularly conducive to a solution, as it seems like his go-to answer is "deal with it."

In the letter, I recommend saying what you said here and focus on your emotions. Talk about how you feel when your step-mom and your father treat you like you're inferior to your step-siblings. Talk about how you feel when your step-siblings invade your privacy. Talk about how you feel when your dad and step-mother don't do anything when your step-siblings break the watch from your LATE MOTHER. (I think it's ridiculous how your dad remains passive and tells you to essentially suck it up when something like that happens. No offense.) Talk about how you feel when your dad doesn't show up to your games anymore. (I can relate to that one; my dad never showed up to my basketball games)

And then list off all the other injustices that you described here. Like, why the hell does your step-sister get more than double your allowance? And a room for herself? Like, I can understand the room for herself I guess, but it makes no sense that she gets 215% (I did that math in my head really quickly, so sorry if that number is wrong) of your allowance. Talk about how your step-siblings are systematically destroying all of your things.

It's also very important to address this whole "sacrifice in the name of family" argument your dad uses as an endgame argument. Sacrifice? Hell, you've given up your room; you've had to deal with disrespect from what seems to be everybody; (they need to put some RESPECK on your name... sorry, I had to) you've had to give up $780 annually from allowance. Meanwhile what has everyone else sacrificed? And in the name of family? Like I get that concept, but (from what I know from your post) your step-mom regularly alienates and rejects you, so what's really happening is you're sacrificing all of your stuff just to be shunned like an outcast from your own family.

But the most important part is that you need to present yourself as mature and stand your ground. Your parents will (from what I can gather from this post) be eager to run you over and dismiss your concerns, but you have to be adamant.