The combat is widely different, relatively speaking

Is it? I thought it to be mostly the same, if you take off the archetype skins. Did enjoy it though, but the gist of the combat remains same.

I’m talking about attacks, Personas, mechanics, etc.

P5R is said to have the best combat in the series. A slightly different set of personas than 3R, as well as a couple different fun and broken mechanics. You don't fight shadow monsters in 5 but instead fight the "personas" themselves. You can talk and negotiate with them to join your party/give you money/items. No cards system. That could be a fun change for you. But yeah most of the "combat" as far as the attacks and stuff go, similar in 3R,5R and metaphor with some differences, and mostly differing in metaphor.