I got it! Boss rush was super fun!! Only had to do a whole new playthrough to unlock the last “undiscovered” boss… (more details in comments)

Soo after I fought and defeated the elder naytiba last time, I reloaded my cloud save and did the other ending and started a new game+ which meant the my save file never counted that I had that boss discovered. But I got the platinum so it didn’t matter. But now I had to defeat the elder again by doing a new playthrough just to make him show up in boss rush… Oh well, at least I got to rock the new update outfits while doing so!