I'm a longtime fan of your work, Josh, as you may know. Good work here.
The game seems to be pretty awkward when handling blocks that take up more than one space, that is to say the 3x3 and 5x5 blocks. The game counts only the center of the block as occupying a space, so you can still place blocks within the bigger block and they'll appear either above or below it even though they're not. I'm not sure what the best way to fix it is. I think what I want to happen is for the 5x5 block to actually create 25 blocks so that you can chip away or replace pieces of it rather than just leaving or removing the whole thing.
Then of course there should be an option to erase multiple blocks at once. Like maybe you could see a transparent cube that you're moving around that represents the area to be deleted.