I would argue that the Splatoon 2 and 3 DLC is the best content in those games, far outstripping what the main SP campaigns were doing (and certainly better than the MP, but I've never been a big Splatoon MP guy).

Playing through Raiders, I'm not loving that pretty much the ENTIRE focus of the game is on the not particularly special Splatoon series combat instead of the platforming, but the game's been decent enough so far.

I'll admit that I haven't really kept up with the news on this one too much. But is Raiders supposed to be like a Roguelike or Roguelite game?

Not in the slightest. You have a persistent character level with skill points, perks, and gear you unlock. The only thing vaguely Roguelike about it is that the game's kind of hard when played Single-player (but extremely easy when played in co-op), so it's easy to get overwhelmed by the swarm and lose your lives. Losing all your lives kicks you back to your ship with everything you'd already gathered up to that point, and you have to start the level over.

This game is a looter shooter, like a level-based version of Borderlands.

Oh yeah. Looter Shooter. I never thought of something like Destiny 2 (my main point of reference for that type of game) when seeing this in action but now that you say that it does seem kind of obvious.

However, it seems rather ironic that for a game being billed as the first Splatoon single-player game that it works best with multi-player (co-op).