Axel mode unlocks a couple new mechanics and 3 new characters/classes, but it's overall rather short (albeit it can be a bit hard if you just played through the story in Adell mode). Once you're finished with it, there's not much reason to go back, so you can keep looping Adell mode over and over again if you so wish.

You can unlock/pass some bills for side missions that you can try to complete, though the difficulty is kinda all over the place (and some of them requires you to be on certain chapters or before a certain point in the story). An easy one that I recommend requires you to farm Defender of Earth cellphones (use 3 of them on 3 different instances/stages to unlock the bill for the mission. IIRC it's one of those bills that require you to use Adell as a senator). The mission rewards you with an Accelerator, the best "shoe" item in the game, and you can grab one per (Adell) playthrough. You can technically get another copy through the item world, but its legendary rarity isn't guaranteed and it takes longer to do so (as a bit of trivia, the devs didn't even expect you to do so, the Dark Record lists Accelerators as only having a legendary version).

Other than those and just grinding stuff in general (levels, innocents, pirate maps, leveling gear etc), you can try doing the Dark World stuff. These are alternate stages from the story mode with a certain gimmick in them (it has a bit of a "puzzle" element to it). To unlock them, I heavily incentivize you to just look it up online, some are pretty obscure (to the point that many guides might not have accurate information/list different requirements for the same stage). While not required, I advise you to do the Dark World with all story maps available so you can unlock/clear them all in one go, so if you already started another playthrough recently, it might be better to hold off for a while. Oh, and I'm referring to the Dark World in Adell Mode; Axel Mode does have DW stages, but there's no reward for clearing them, so other than some potentially good early grinding maps, you can ignore them.

Thanks so much!

Glad to hear Axel mode is short

With any Disgaea game, the first credits unlock is only the beginning. You can usually finish most of the 'main' stories at maybe level 100 or so without really using Reincarnation or the Item World, but that's usually only like 10-20% of the actual content in the game.

Honestly the achievements are pretty good targets for goals for postgame and replayability. I did 100% achievements in ~110 hours (although probably a good 20-30hr of that was grinding for Pringer X Go by perfect leveling enough items to do it).

Axel mode you should do early because it unlocks Magichange (and then you can get Magichange 2 from an Item World rare room).

Yeah I agree, Axel is a terrible and annoying character, but its mercifully short with only 4 chapters with less than 20 battles total, and it doesn't take very high level to beat, the highest level enemies are only level ~250.

There's a lot of stuff to do, you have the Dark World levels, sure some of them are terribad but since there's a Dark World level for every single regular level, that's to be expected.

Other than Dark World you have all of the bonus Dark Assembly bosses, the extra endings, then the Land of Carnage unlock, and finally the Pringer X Go Unleashed fight.

Land of Carnage is a bit of a grind though, and Pringer X Go requires a huge amount of grinding.