I need to get back to P3P - which I will, don’t look at me like that Zero - but what I’ve played of it I enjoy a lot so I can at least say that as someone who wandered into it blind on mechanics
I think P5R and P4G both do a lot to improve the base games, and I'd recommend playing them. P3P ... yeah, as much as I love the phantom thieves and a lot of the ideas at play, I don't think anything tells the story as narratively or thematically cohesively as P3P. You're going to miss a lot of the QoL stuff from the later games, but story-wise ...?
Ie the eternal pit of wasted mp and turns that your healer SHOULD be using on heals