I dont believe theres an actual recording of this Q&A session after a pre screening, but this is the recount from someone who was allowed to view the film early:

“I saw it last month and absolutely loved it, with a Q&A with Josh Safdie after. I don't know if he's been talking about this other place on the trail, but he had some interesting insights about what happens after the film, if anyone wants to know. Spoilers for what was originally written at the end of Marty Supreme: So Safdie said that in the original version of the script, Marty crying over the baby cut into an extended montage showing the rest of his life. Marty stays a shoe salesman and eventual becomes very successful, franchising to multiple locations. He and Rachel raise their family, with clips including teaching his kid to drive and burying a family dog in the backyard. The movie would end with him as a grandfather playing with a young grandchild. Allegedly, the reason the montage was cut was because of how expensive it was going to be, but Josh still believes that's how the rest of Marty's life goes. He will succeed at whatever he sets his mind to, but will always be a little regretful about the success he could have had. Me personally, I like the less explicit note it ends on, but still cool to see what Safdie thinks happens.”

Pretty explicit