Most people would scream Fire Emblem at you, or point you towards an indie SRPG, but yeah, Disgaea is Disgaea - or NIS is NIS even, as a lot of their games have a focus on heavy grinding and getting things set up just-so - and there isn't really anything that directly competes with it. Compile Heart titles have a similar sort of aesthetic, but not the gameplay. Offhand I can't think of anything that compares gameplay-wise outside of Etrian Odyssey, and Nexus was the last one we'll probably be getting for a while. Well, I suppose there are MMOs, but it's just not the same. It's not a complete experience with a definite beginning and ending.
That'd be the platform, no doubt - the Vita has always done well in Japan, but Sony's complete lack of support for it killed it in the West. If they had released it for the 3DS, or were to re-release it now for the Switch, it'd probably do much better than it did previously...although I haven't seen sales figures for Disgaea on the Switch (either of them) so I couldn't say for sure. But judging by the general trend for things to sell significantly better on Nintendo consoles, it seems like a wasted opportunity. It wouldn't take much for NIS to make up some ground on the Switch...although their translation debacle with Ys VIII (which was amazing by the way) hasn't earned NISA any points with Western extremi--I mean, fans.
It's been a while since I played an anime-style game I could really get behind the same way I used to, honestly. I still have Labyrinth of Refrain to play and that looks amazing, but everything I've played recently has fallen short. I'm looking forward to Mary Skelter 2 but that'll be digital-only, showing a distinct lack of commitment (although it is dubbed, thank god) and Atelier...Atelier is dead. It's getting harder and harder to find good games with just a similar aesthetic, never mind similar gameplay.