I liked this game a lot! I thought it had a nice gameplay loop and expands on it well.
The boss mechanics kept things interesting and engaging. I think it is interesting how it becomes a genuinely different challenge with each boss.
I played this game pre and post patch and I think you really made some good changes in between patches. Pre-patch was BRUTALLY difficult. But Post patch seemed a lot more balanced.
I do think that the scaling for the blocks, where blocks with more tiles have a higher cooldown is pretty extreme. I usually ended up picking the short cooldown blocks because they seemed like better value. You often would not need the extra tiles of the larger blocks and the smaller blocks were more flexible anyways. Also, since the upgrade system always reduces blocks by 1sec, the shorter cooldown blocks are better with upgrades.
That being said, I still enjoyed the upgrade system a lot. It really tied together well with the coin gathering. It's a nice bit of game design to tie the systems of your game together so well. The gameplay is actually quite cohesive in general. I think the plot premise ties with the gameplay so well the game in general actually feels like a very cohesive and neatly wound package. There is something wonderful about just how much the plot and gameplay just "make sense". Of course this is how Kobo would attack people.
Minor nitpick, when playing on harder difficulties and dying a lot (or pre-patch), I found myself wishing I could skip the Boss's intro cutscenes.
I think the game also puts emphasis on the correct things that really contribute to this gameplay. I think it's shocking how quickly I just adapted to the notion that Calli is just one frame of animation and didn't really mind. My mind was more interested in the hazards and environment anyways. As an animator... it really gives me a minor existential crisis.
Oh, and also good song choice.