Wow, thanks a lot for the heartwarming feedback! I'm really glad you liked how things went.
I had trouble indeed sizing how hard the last levels are at the time. With hindsight I do believe you that I should add some more levels in-between to smoothen the difficulty curve. As for the game improvements you suggest, these are great ideas. I particularly regret not managing my time well enough to add at least some history listing the last attempts for reference (and I should take your game as a model since it is a great example about how a good UI adds a lot).
Thanks again for all your supportive words!