The ability to make those small changes is great! A bit buggy on desktop, the points never reset even between games.
The speed change button should use more than just color to differentiate speeds, right now the orange/red is hard to tell and I'm on a good quality desktop monitor. With night mode on a phone with low brightness, orange and red are going to look very similar. Doing what other games do, having the button change from 1 arrow -> 2 arrows -> 3 arrows to show the speed up would probably help.
I think you have a really solid core game play. The next obvious step is to differentiate the towers some, right now there is a straight forward progression and skill point unlocks just make everything obviously more powerful. What if instead you kept with the gambling theme and for example had unlocks that made a certain pair special. 2 aces -> snake eyes, long range but super slow to fire, or triple (lucky) 7s making surrounding towers occasionally fire an extra shot.
Not necessarily those ideas, just something that makes spending skills feel meaningful. Right now unlocks feel like "oh yeah, I'll do better because math" but there isn't anything to really explore in terms of strategy.
Loving the updates so far though, great job!