Very fun demo!
Just beat it on easiest difficulty. Will try the other ones next but I want to write down my first impressions here.
I really liked the changes of pace, from calm beginning to action packed and back to calm at the end of each level. Game feel was good and the controls were precise. Sometimes though I got hit by the red missiles and it looked like it shouldn't have. Other than that the hitboxes felt fine.
My biggest criticism is the visual contrast. I love the art style and colors but it was sometimes very difficult to distinguish between dangerous moving objects and the background. I feel like in a game where there is a lot going on like in this one (which I love btw) it should be easy to see at a glance what's to be avoided and what isn't. That way when the screen gets more and more crowded like what I'm assuming you want to do with later levels, the brain can easily filter out objects and details that are not dangerous. I think the hazards were perfect, it's mostly because of the background that was maybe a bit too busy. One thing that I think could maybe work to tackle this without having to make the art itself less saturated or dull, would be something like auto exposure, where the background gets slightly darker the more bright things are on screen. That way it scales well with different amounts of enemies and the calmer parts of the level also don't have to be changed.
Another thing that kind of plays into the contrast thing is that it was a bit hard to see the mouse cursor at times, especially when it was next to the white toothpaste. I had to look for it a few times while trying to dodge the laser :D Could maybe add a custom cursor that people can enable/disable in the settings or something.
It would be cool if there were ways to get extra money to open up some more options for upgrades. Right now there is only a limited amount of upgrade combinations since the money you get every time is fixed. Maybe you could get a small bonus if you are fast enough, or if you beat the level without taking damage.
When I first encountered the missiles I thought it would be fun if you could somehow damage or destroy them. Something that is risky to pull off but rewards the player for getting close without getting hit. Or maybe toothpaste could have an effect on them, like make them short circuit and explode after a few seconds.
All in all I really liked the game and the demo ended just when I was most excited to see what comes next. Sorry if I was a bit nitpicky but I know from my own experience how important it is to get constructive feedback on your games without any sugarcoating, so I tried to be as honest as I could. Hope this helps and good luck for the rest of the project :)
Now onto the harder difficulties, time to clean some t e e t h