This Mosa Lina x Metroid kind of gameplay is absolutely wonderful, and I can't wait to see this game at its full potential. However, it definitely needs some tweaking before you can ham on adding new content.
First of all, the game's initial difficulty spike is immense. Sure, learning new enemies and mechanics comes with the territory of practically every roguelike, but having to learn about several new elements in every room without any tutorials or proper introductions is brutal. Just off the top of my head, several solutions come to mind, some better than others from a balancing/time investment standpoint.
First, I think moving Wyrm 1 to Wyrm 2, 2 to 3, etc, and then adding a new, much tamer starting level would help a lot. Each room of the new Wyrm 1 could keep things simple, having only a single enemy or mechanic for the player to be eased into, with maybe a set of switches that need to be flipped before the player can proceed. If you aren't keen on that, then taking out a significant portion of Wyrm 1's spikes would be another good option. If neither of those sound appealing, then spawning an extra, vertically-oriented movement item at the start of each run might be a good way to go. Heck, you could let the player choose between a handful of options, for even more replayability.
Speaking of items, the way that they work is downright confusing at times. Sure, the movement items you get have nice explanations to them, but nothing else really does, especially the ones in the shop. Half the time, I forgo buying health to save up, only to buy an item that has no explanation text to it, or even a perceivable benefit.
Also, assuming you don't plan on giving the player a starting item like I mentioned above, I feel like altering Wyrm 1's item pool to only have better, more vertically-oriented items is a must. Every time I get something like the dash or masochism items, I immediately restart the run, and I wouldn't be surprised if other players did that too. Given how punishing the player's base movement and physics are, the extra safety net is basically a requirement for a good run.
Despite all the tough love I just had for this, it all comes from a place of respect. I myself have toyed around with the idea of making a game just like this, so seeing it come to a reality is a dream come true. I don't know how open you are to idea of incorporating suggestions, or even bringing on an extra developer onto the project (I have experience with GMS:2, and I'm pretty decent at pixel art too), but I'd love to contribute to this either way.