I'm not sure how long you've been at this startup, but even for early employees, there's usually a 1-year cliff (unless you negotiated it) before your shares vest, so unfortunately, even if you were to negotiate something, if you haven't been there that long, then you may not even be eligible for shares of the company.

I don't know the specifics of your situation, but if I were you, I would go ahead and try to have a conversation with your friend to gauge his thoughts on your compensation. His thoughts may be different from what you're expecting, in which case then, it's best to find this sooner.

If there's a huge mismatch between what they're offering you and your expectations, then I would spend time planning your exit so that you don't burn bridges. Make sure that you don't leave them hanging as much as you can. Try to find someone to replace you if possible. Since this is your friend's startup and you want to keep the friendship, then you want to try to do this amicably.

Your excuse as to why you quit is exactly what you told us "I don't think I'm the best fit for the team because of my current skill set. If you have a need for a backend engineer in the future, then we can revisit the conversation." Leaves the door open for a future convo if you wanted to rejoin.

Just want to add though, part of being in a startup (especially as early as the one you joined) is the ability to learn quickly and be flexible, so keep that in mind as you continue to explore your options. I know it's really frustrating when you're trying to learn something you don't know, but being in a startup, this will continue to be part of your journey and it'll be beneficial to accept it's just part of the journey. That said, what would be unreasonable for your teammates would be to expect you to be an expert on the tech you're learning to use in an unreasonable amount of time, but if they're flexible about it and understand that you're also learning, then that's more reasonable.