tldr. Great design. Good potential. Fix collision box and Iframes, then dash direction, add platforms., maybe add knockback of player too.
First, let me say, I have been on and off game dev and absolutely love playing all sorts of games. I will give ideas and feedback as best as I can for a player's viewpoint and from a developer's viewpoint.
Also Denny's Guy comment is pretty spot on.
First, I agree that the game window should be scaled, the scaling would also make your game accessible on multiple devices and would make things easier in the future too. That should be automatic whether it is on the web or when downloaded.
Dashing did seem to have a some bug with direction, which may be some issue with keeping track of the last key pressed and combining shift and A,D,Left,Right, not sure. Honestly I never quite figured it out.
The design of the boss and player were pretty nice. The boss seems awesome and could have more vivid phases, so I look forward to any development there. I won't beat into the mechanics or if there are other components of the game since it is only a beta.
With the game mechanics, the player should probably be able to attack up (kind of like hollow knight in that manner). *I think I understand why the health seems to melt and feels inconsistent. There are two main factors. First, there are no invincibility frames. The bosses projectiles can hit the player one after another which melts health. It should only be a short iframe, like .2 sec, but enough that the player should dodge and should take damage at maybe the first and last projectile (if they get hit of course). When implementing Iframes, I like adding knockback to the player to help them get out of the situation and it sort of resets the player so they can't abuse the iframes. Then, the next issue is that the hitboxes are inconsistent. Sometimes the damage of the projectiles does nothing. Maybe backend it subtracts health, but does not display the loss in health. That could lead to a sudden loss of health unexpectedly.
I think the Variable jumping is a good mechanic, but different height platforms and the different height of the boss teleporting (I think this part was done) would be almost necessary.
Lastly, I love the artstyle. The boss is awesome and a nice design. The player could be more detailed, but the dash has good effects. The projectiles could use a bit more variability in design since most look like tetris blocks. The bridge could also use a little less arch and more 'bridge' when connecting arches. Otherwise, the game seems to have a good developer and artist capable of making an amazing game. Good luck and can't wait to see its future :).