This comment above about difficulty made me wonder if there are more interesting things to do with the unclear-able blocks that would both let you ramp the difficulty and achieve more of your vision. A few off the cuff ideas:
* enemy blocks that are clearable if you make a row and a column through them * enemy blocks that require to be surrounded by n deep blocks to resolve * enemy blocks that automatically pop into single cell holes * enemy blocks that grow by infecting neighboring blocks over time or the more other rows you clear * enemy blocks that require multiple row clears to resolve
the thought is having a few types with each enemy block needing a different strategy to resolve would give you more levers on how to control difficulty and give you more variation to the strategy and more agency to defeat them. powerups is kinda a deus-ex-machina currently. you dont feel earned satisfaction when you clear an enemy block.