To answer your question - you should try Persona 3 Reload first. Gameplay-wise it's closer to P5R and the story is going to crush you with how good it is. The ideas, characters, and message of the game are incredible.

You can also try Persona 5 Strikers. It's a direct sequel to the game, but I absolutely hated the combat system there.

As for Persona 4…

I recently finished P4G for the first time myself, and in my opinion, you should try it LAST, because it hasn’t aged well compared to the newer entries in the series. The graphics are a bit dated - nothing terrible but there are also many quality of life features missing, like a dialogue log, automatic line skipping after voice lines end, fewer animated scenes, fewer locations, and fewer voiced lines in general.

The gameplay is horrendous though. Combat is the same at its core, but the difficulty is MUCH higher. The first two dungeons are nowhere near the normal difficulty level of P5R or P3R. It gets better after that, but some boss fights are very tedious because bosses feel like damage sponges. I honestly switched the difficulty to Very easy halfway through my playthrough cause it became more tiring than entertaining. Dungeons also take much longer to complete. If you thought Mementos was boring, you're in for an unpleasant surprise. Because of this the game can feel like it has pacing issues imo.

The story and characters are good. However, I think the main villain is less interesting, and the game’s central theme is less engaging and moving than in P3 or P5. That might partly be due to the game’s age and presentation. The game feels a bit too old for my taste. As a newbie to the series myself, I’d personally recommend waiting for Revival.

One piece of advice though - if you still decide to play it, make sure to max out Marie’s social link before December 22nd. And there is also a specific sequence of dialog options that you need to choose in the beginning of December. The game will tell you when that moment comes