There’s been no pressure for me to stay, but I feel an obligation personally

If you're meeting your timelines and your manager is fine with your work, you need to get over that feeling. I used to be that way when I was younger, but as someone else said the work will always be there.

Putting in extra time when nothing is on fire is a sucker's play and companies (especially in America) bank on that job insecurity/money insecurity for a solid chunk of their revenue. If you love your work and you want to put in extra hours that's your call, but if you're doing it out of obligation/guilt rather than any actual output deficiency then you need to sort out your own feelings. There will always be more work, and you're not getting paid extra for doing that extra work. Go home.