If you end up liking Eternal Punishment, definitely check out Innocent Sin because the games are directly connected in a deeper way than most direct sequels.

Persona levels are not much of an issue in this game as they only go up to level 8 and as long as you have them equipped on characters who have great comparability its not much of an issue to power level them quickly. Personas level by just using fusion spells which is what the game wants you to do anyway since they make battles much easier. Its only trying to bother with mutations that can get tedious but if you don't care about any of that nonsense then you can breeze through the game fairly easy. The story gained Greek Set Ultimate Personas are more than enough to beat the game.

Eternal Punishment's plot is pretty dark compared to other entries, I think only P3 comes close to meeting it. Innocent Sin is a bit goofy with some dark undertones until the final act when the game gets pretty dark which then leads into Eternal Punishment.