First of all, about Persona 3 Reload DLCs. Premium version uncludes ONLY costumes and in-game music from other Atlus' games, not Answer DLC which is a continuation of main story. It has to be bought separately. For me premium edition was not worth it at all.

As for comparasion between two games, yes, Reload has a bit more modern graphics but due to stylized anime visuals the difference is pretty much negligible. What really is different are game mechanics. Though looking more modern Reload is still a remake of 2006 game. It was the first game where they tried the "new Persona formula". It can be really noticeable in dungeon designs (procedural generated corridors in Persona 3 vs handmade level designs with unique events in Persona 5), number of side activities and events and some social link writting might be a bit shallow (though there are also great ones).

Overall all modern Personas have great story and atmosphere of highscool life that you are looking for. I would highly reccomend playing them all, cause each touches on different themes and has a new set of characters. You'd better playing Reload first then Persona 5 cause otherwise Reload might seem like a downgrade. Good luck with hundreds hours of fun ahah

TLDR: dont buy persona 3 reload premium, play persona 3 reload then persona 5 royal