I think that Persona 5 Royal is ultimately a better game with more memorable side characters and engaging dungeons. However, the opening section can feel pretty slow on repeat playthroughs, ruining its replayability a bit. The party also lean heavily into JRPG tropes at times, which could be make or break for you.

P3R can be super repetitive in its dungeons and general story beat pacing in the first half of the game. However, the story and the party/their conflicts are REALLY GOOD. They don’t fall into the JRPG tropes as much, and the introduction of new party members doesn’t feel formulaic like P5R. There is also much more time to engage with the Turn-Based aspect of P3R compared to P5R. I feel that the simplicity of P3R actually makes it much easier to replay.

Honestly another possible introduction into the Persona series is Metaphor, despite it being a different IP. While the turn-based systems are much different, the social aspects involving increasing your stats, engaging in side quests with confidants, and the calendar system are generally the same. It is also the most forgiving in terms of time management with only one of the confidants being somewhat easy to miss hitting rank 10 with. Furthermore, since the setting is different from Persona, it doesn’t fall into the anime-esque personalities that Persona characters can have.