As someone who also made a game about assembling a gameboy (or "JamBoy", if you will), I was excited to try this out! I see Robert mentioned the black space (leading to the A button) a little hard to find, but I thought it was designed rather nicely with the larger space on top telling you to find another way in! An important distinction though, I played on browser, not a DMG.
Unfortunately though, I couldn't make it past the boomerang bros north of the castle. I would kill them and die when walking through the path by what seemed to be an invisible boomerang of their's. Also, I don't know if this was intentional or not, but I would also have to regather the B button to use the boomerang every time that I died.
Lastly, I was unsure on what the item in the bottom left was. Perhaps it's explained later?
I like the idea a lot! Limiting the player to their abilities until they gather more buttons takes the metroidvania style design to a new level! I would love to see an more with not only an update to the boomerang bros thing, but to expand more on the limited abilities idea - such as making more challenges like the initial only-using-up section but really exploring all of the different ways you can build that out.