I read other people's comments, then went back and found the unlocked onsen and the two trial areas. I thought the trial areas were really fun, especially with the check point so close. It kinds of reinforces one of the comments another player gave that this game feels more appropriately like a Celeste-style game, a series of rooms each of which is its own challenge that need to be overcome in sequence. But yeah, with the trials done and the extra enegy under my belt, I was able to go back down and defeat the Oni. Good times!