Like the art style you guys got going, works really well. To address your questions:
1. I think movement is fine as is because of how little space there is to navigate. I think a better thing to improve would be to improve enemy behavior more than movement. Since this plays like Galaga, you should make your enemies telegraph attacks more and give the player more time to react. With those improvements and how movement is currently, I think that'll make the game feel really good.
2. Projectiles seem alright so far and I think maybe a power up or ability for a full auto fire would be helpful too.
3. I think your player health was fine, I didn't have any issue with it.
4. Definitely the UI scaling. I also had some similar issues like others with how the UI was scaled. A lot of the informational stuff was cut off pretty much. I could still play the game but I couldn't really read the other info you guys had. For reference I was running this on a 1440p monitor so I think making your executable windows scalable would help this. I think also cleaning it up and getting it looking better would do wonders as well for the look of the game too. I think for the final release that's a good improvement to go for.
So far it looks good and I think you guys will have a good product come final release time. Good luck on the rest of the development!
-Micco