It would be nice to have some kind of clue as to where the demo/prototype ends. I got stuck and couldn't tell if I was missing something or if that was the end of the content. (I got the ... "megaphone"?)
After reading your updates, I feel like you're on a fine trajectory. I have also experienced the struggle of NOT being a natural at pixel art, and I like the Godot engine, although I only worked with it briefly. Congrats on your progress with the art. That is a big improvement! As for grass, let me tell ya, I've struggled with that at several different levels of graphic size, and it always feels like a struggle to make grass look like grass and not some sort of messy argyle sweater. In an 8-bit format, what you have there is perfectly acceptable: mostly just green with a few flowers, shoots of taller grass, etc. here and there. My guess is, the map feels bare because you're thinking of Zelda 1, which was very full of bushes and other things leaving the player less room to move about. I don't see any reason to feel like you have to fill every screen to the brim with objects, as long as there's enough there to give it some aesthetic character. I am curious how exactly you do most of the art and tiling. For the record, Paint.net is slightly more useful than the basic Microsoft Paint, but not as complicated as more advanced tools, and it's still free. I know there's better stuff out there for pixel art, and I think some of them have a proper tiling feature.
I recognize that, not being sure what you have already changed or have plans to change, some of my advice/suggestions may be superfluous. That said...
Making the hit detection on the corners of walls slightly rounded would make it easier to walk through single space gaps/exits. It was a slight struggle to get out of the graveyard, because the character really has to be facing that exit very straight.
Having the A and B buttons be Enter and the actual B button felt very weird and makes it equally awkward to play with either the arrows or WASD. I found it odd that the key was a "Quest Item" but also equippable. I'm used to the term "Quest Item" implying a thing is mostly representative, but I suppose that's not a hard and fast rule.
I'm looking forward to seeing more in the future. You're off to a fine start. There's certainly nothing wrong with getting all your art laid out ahead of time, but if you start to feel burned out, I recommend you take that as a subconscious que to switch to another aspect of the game's design. Best of luck!