Got to round 36(no failing to meet goals) then decided to leave it there. I didn't blow the points out of the water, but at times I was doubling/tripling the required score. By the last round it was pretty close.
I'm somewhat surprised you're doing a Inscryption by having a dark moody room where you play the game. Mainly since the game doesn't really have a horror feel at all, though it does have a hell set dressing I guess. Besides looks, it's also very casual since it's mostly spray and pray that you get enough points.
When the boss round says you can't use the first item slot, it means you can't click drag the item inside it. The only way to swap it out is if you drag another item onto it. I assume that's a bug, because for instance if you only had 1 item you couldn't move it into the 2nd or 3rd slot if you wanted. Speaking of moving, I somewhat expected that I could move the special pegs around on the board. If you don't want that, then I would have expected to choose which peg gets the upgrade.
Due to random chance I only got a couple Item Shops in the early game, so at the end I only had 3 items. Not sure if I could have upgraded to 4 or 5 slots or not, which would make the winning margin even easier.
It took me several boss rounds to realize that the difficulty increase was for the rest of the run, due to the name it sounded like a 'this set of rounds is the boss' or something. Maybe small, but consider choosing a name more obviously for a creeping difficulty, like 'enter the next layer of hell' or some such.