I think perhaps just tightening up those early zones to make the gameplay a little more focused may help. To give an example, for the first door that required a key I realised I could just dart past the enemy to grab the key then rush to the door instead of engaging in combat. Which I think I did because the majority of the level up to that point had been about dodging threats (the arrows) rather than facing off against enemies.
I do like your idea of making everything hit harder (both player and enemies) as that would up the necessity to really take ones time and engage meaningfully in combat--and would force me to take on the enemy before grabbing the key, for example.
Unfortunately I didn't make it to the boss so I can't comment on that, but I like the design which I did see.
Ultimately, I'm not the ideal tester for a game like this as I tend to shy away from 2D experiences so do take my wordsd with a grain of salt.
That said, I really do think you have some great base mechanics and a solid art style which are the strongest of foundations for something truly great!