I've removed that part completely now, and added a leaderboard to let you see how fast it takes you to beat the final boss at a certain rebirth e.g. took you x time with 0 rebirths, y time at 1 rebirth and so on to give you the ability to do that. Feel free to give it another shot and see if it helps at all if not, I'll do a heavy rethink of how I go about it. If that fails I think toning down the monsters by 20% for example and then having you unlock harder difficulties on first completion would be a good alternative.
Thanks for the feedback either way, gave me a lot to think about. Part of why I posted it here is to get it in the hands of people even if it risks getting a negative reaction to be able to start balancing and seeing what works and what doesn't/