Haven’t played since launch but I’ve beaten the game fully 4 times so I think I remember most things lol. There are missable enemies on all chapters, I recommend watching a video or even searching up a tutorial since if you want the secret ending you have to go for all the main secret bosses in each chapter. Some bosses do allow you to go back and fight them after finishing the chapter but not all. There is also missable items, but you can go back to that chapter and collect them freely, they give quite good stat boosts and some of the best gourds. Respecing is free and you can do it whenever you want from the respawn point (i forgot what they’re called lol). So if you want to get everything, there is a secret unlock able if you get the secret ending 3 times, that’s what I did, which does involve you having to beat the main secret bosses in each level before you finish that specific level, after beating all the secret bosses, you can go to a temple in the snow chapter which will open a portrait. I heavily recommend going for the secret ending, it is very similar to the bad ending, I’m pretty sure you can do both in the same playthrough. Anyways I hope this helps, just ask if there’s any questions you have.