Chandler being able to afford paying for rent AND providing for Joey is also incredibly unrealistic.

Chandler’s job was just made to be some generic finance sector job, right? It’s definitely possible even today, but he’d be working a lot more hours. You’d never see him on the show.

Ross being stable even as a PhD grad student seems a lot more unrealistic to me. He even loved on his own. But maybe it was family money.

Ross wasn’t a grad student though. he was a PhD researcher + professor. back in the 90s, that would’ve been a decent gig.
I was under the impression Ross was still in a PhD program the first year, working at the museum seems like a gig for a PhD student. Worrying about the museum displays and stuff like that in season 1.
nah there was a whole episode where people were banging next to where his dissertation was in the NYU library. he ends up banging a girl that was interested in his paper while policing the library next to his paper.