Thanks so much for playing, and your feedback! You're not the first to ask for gamepad support. I'm planning on adding that in a future update. (I actually prefer keyboard controls, but I can't only cater to myself,, haha)
With the slide jump, the first few spike pits should be able to be safely crossed. The final one (after the seal) indeed requires combining it with the uppercut, and leads to an optional (and skip-able) collectable. Those were intended as bonus items for players who enjoy tougher movement puzzles. In hindsight, that one in particular may seem like it's part of the "main path"... I'll have to consider how to handle that better in the future.
As for the end boss, spamming the uppercut to smash her bell is one way to beat her. HOWEVER.... (mild spoilers for any new player reading this?) .... breaking the bell will lead to the bad ending. There is a more challenging way to defeat her that will give you the good ending. (If you're interested in seeing it, you can use the "Continue" button on the title screen to pick up from the final boss stage, so long as you haven't started a new game)
Thanks again for playing, for taking the time to comment! I really appreciate it. :)