Saw you mention it here and also in the Gamedev Discord and wanted to give you my thoughts on the question of how difficult to make the game which you're facing right now. Since some people are saying it's too hard, and some saying it's too easy, you're probably close to the sweet spot of difficulty right now. (Of course, the game can naturally increase its difficulty a bit in each level as it continues and especially as you add new powerups and/or stage hazards).
You bring up Hollow Knight as an example of a casual platformer, and while this is true most of the time, it also has some incredibly difficult late-game optional areas, challenge arenas, and optional bosses. This is a classic platformer tactic. (A much older example would be Star World in Super Mario World.) Since having meaningful difficulty option settings can be difficult in any genre, but especially so in a platformer, having special challenge bonus levels and more difficult hidden levels/areas can allow you to keep the difficulty standard for the rest of the game and players who want a more casual playthrough, while allowing you to present real and optional challenges to players who crave a more hardcore challenge.
Of course, none of this is mandatory--just some suggestions! It's your game and it's your choice what to do with it, and I look forward to hopefully seeing more of it after the jam is finished.