Seeing as you get way too much Rainbow Balls later on, i'd say that increasing the RB cost for Plinko every tier, as well as adding a RB Shop (not sure what could be there) could work very well
Also, selling frequently seems to make you have negative balls???
The controls are really good (but a bit hard to control), but i've kinda felt that both the "restart" and reaching 0 on the time are very punishing. i'd suggest either a section select when entering the level, or making the section automatically restart once the time runs out, rather than immediately kicking you out of the whole level
Edit: nvm this would make it too easy, but it's still a bit frustrating because of these issues
I'd reccomend making the placeable field a bit more square, as it is currently a bit difficult to place a good defense on the vertical edges.
Making the field slightly bigger every couple of waves could be a thing, if there will be enemies that need a lot more damage than usual.
Having to hold the mouse just to collect coins is a bit annoying, i actually first thought you had to click on them. Maybe make the coins just have to be hovered over?
And lastly, a fast-forward option or an indicator how much is left until the next wave could be good, instead of having to wait without knowing when it starts.
Still a very good game tho
(I accidentally made this tab the image i was trying to upload which made me start from the beggining)
