employer started demanding constant unpaid overtime
There is a difference between your employer demanding OT and you feel obligated/want to put in additional OT. Either because you have a vested interest in the project your working on or your the only one putting in the minimum when everyone else (including the founders/owners who are developers) put in OT.
Similar situation as you (4 years experience with undergrad). I love my job and the company/people I work with, however, I feel I do not carry my weight if I do not put in at least 45-50 hours a week. The job market is strong enough to easily relocate. But the OT is a small price to pay for who I work with, where I work, and what I work on.
I only have 4 years in the workforce and in that time I had a few bad places to work that had a direct effect on my health. I can only imagine after 20 years of crappy jobs you finally find one that is what you have been looking for. The only thing, OT is an unwritten rule. I already made the choice to go with OT to at least have the satisfaction and enjoyment of the work I do with the people I work with.