For me, anything 25 FPS or higher is 100% fine and I’ll be enjoying my time. I never play competitive online shooter games ever, though. All single player ones like GOW and the likes. I game on a 60 Hz 4k monitor. GPU is AMD RX 6600 alongside Ryzen 7 5700G and 32GB RAM. My games are set to meduim most of the time at 4k. Demanding titles are on low. Surprisingly, GOW and GOW Ragnarok are both set to ultra and I still get around 40ish FPS.
I too grew up on machines that were mid-low range and was constantly asking more of them than they could handle, so I learned to stomach pretty miserable FPS. In the end though it’s highly context sensitive - the less movement (and in particular camera movement) the game has the lower the frame rate you can get away with.
As a general rule I would say 25 FPS is the absolute lower limit, but around 40 is probably more in line with your “this is fine and I’m going to have a great time” definition. However, for something like a fast paced shooter it’s more like 60 FPS minimum.