I'm honored that you thought so highly of my game XD . Honestly, you can do most things in post-game except for revisit the story cutscenes. You might lose the divine buff though if you take too long. I highly doubt that anyone has figured out how to fight the secret boss yet. There is a clue, but most people will miss it. I did make a slight mistake with that fight, so it is way harder than it is supposed to be. Or way easier if you have the divine buff. (You lose the hidden divine buff after an hour of playtime).
If you want a minor bonus line, fight the secret boss with impossible mode on. Just don't expect to beat the boss in impossible mode.