Advice on how to quit a freelance gig after already getting paid?

Some important context: I have a full time job, but a few months ago, an old contact reached out to me and asked if I was interested in doing some freelance work for a side project of his.

On the preliminary call, he framed it as a very laid back role with very minimal weekly hours. I told him I was nervous to accept, as my work and social calendars were already quite full. He pushed back on this, promising again that what they were asking was not a heavy lift.

They sent me an MOU (which seemed AI generated - this will become relevant soon). I looked it over and determined the deliverables were within my scope.

The pay wasn’t great, they offered a small stipend. We started in June, and I’m expected to keep working on this project through December (so, 6 months of work when they’re paying for, essentially, 2 week’s worth). However, because this project was for a good cause and meaningful, because I knew this person (sort of) personally, and because the deliverables seemed doable, I accepted.

From the jump, it was clear they didn’t really know what they were asking of me. They were putting things on my to-do list that were beyond the scope of what was listed in my MOU (and beyond the scope of my expertise - I think it’s because the MOU was AI generated). It’s been very confusing and disorienting, and I’ve been finding myself watching tutorials on youtube to keep up. Also, something they did not mention before was that everyone was on the west coast (I’m on the east cost, and they knew this). So, all our meetings are past 8:30pm for me, which has taken quite a toll.

On top of all of that, I unexpectedly started a new job in July (right when the brunt of the work for this gig started). I am completely burnt out and clearly bit off more than I could chew. I can’t handle this project anymore and really want to quit.

Here’s the biggest issue (among others): they’ve already paid half of my stipend (the other half will be paid at the end of the project). Also, because the deliverables haven’t been clear, I have no clue if I’ve done an acceptable amount of work for being 2 months into the project.

Is there anyway I can get out of this? I am willing to pay the money back if it means I can stop this work. I obviously don’t want to leave them high and dry, but I can’t let my performance slip at my brand new full time job.

Right now, I just want to know if it’s possible. But if anyone has any advice on how to do this in a way that won’t horribly offend these people, that would be very helpful too.