Okay, that's great info. Thank you for opening up about it.
I'd say try the hard convo before you cut them out: if it doesn't go well, you are still free to cut them off. More specifically, it sounds like the major point of contention, to sum it up, is she is forcing an uncomfortable her vs your mom dynamic that doesn't need to exist, and if she laid off you two could have a better relationship, but if she can't it's a dealbreaker for even being in your life. That fairly accurate?
If so, that convo more or less from your end needs to go something like this: "I'm at a breaking point when it comes to the you vs my mom thing. I love her, so when you get competitive for no reason and start talking badly about her to my brother, getting mad when I want to spend time with her and so on, the only thing you are doing is pushing me away. She didn't
do anything to you. And I'm done. Either this needs to stop, or you're out of my life. I'd rather we got along, and all you have to do is be respectful. Is that really asking that much?
Dad, I love you but you know this isn't okay. If you're going to expect me to tolerate this, then I'll do what I have to. So don't make me, please. I'm not asking something difficult at all here."
If your stepmom argues the point, make it clear this isn't a debate: you are giving them an ultimatum. Be respectful, or you're gone. And if they want to debate it, then leave. Like physically leave the conversation. And follow through. After a while that feels right to you, you can tell your dad you're open to me ding things if she apologizes or swears to drop the issue, however you want to put it.
If the idea of a conversation like that in person is too stressful, for an alternative consider writing your dad a letter bringing up all this so you can compose your thoughts without that social pressure. Finish it saying you're open to talking about it but you aren't compromising on what you want because this is important to you. Leave it for him to read in some way, I'm sure you could figure out the best way to do that.
I've been in your situation but the reverse, biological dad was the one being an asshole with stepmom being around. We had a big argument close to when I moved out. After I did. I barely spoke to him. Time passed. A good deal of time honestly. And one day, he apologized. Said he should have been better, not made so much out of such petty things. We have a great relationship now. I want this for you.
If they can be reasonable and talk it out then perfect. If not though, do what you are considering. It isn't even necessarily the end of the road, but it will sure as hell send a message. It seems like they are still wanting to regard you as a kid, not take your expectations seriously even though you are an adult and well within your right. Give them no choice but to take you seriously, and whatever happens you'll be better off. I'm pulling for you, I know you can do this. If you don't mind, whatever you decide to do I'd love an update. And here to talk anytime if you need that, we all need someone to talk to when we are struggling.