That's a good sign that you didn't notice the loop! Unfortunately the scene pointer for the underboss is off, so you never actually make it to the final boss. The final boss is actually set to be pretty hard, and then you'd have the option to do New Game + if you wanted to build up a truly broken deck, but with the loop that just happens naturally now.
Glad you liked it though! We really had to hustle to get all those dice types done (which is why there are so many other bugs, lol).