Wow, that's a lot of polish in a nice little game. It looks good, has nice music, and a good crunchy sound for when the blocks are erased. Nicely done.
Also got locked in an island with the AI - but having done a ton of these sorts, I know just how hard it is to get the AI right and it takes forever to get all the degenerate cases out of the solution space. I'm wondering how an alpha-beta pruning tree would handle this - might be a little rough since the actual solution space is so large (4 moves + 25 potential erasures would grow vast quickly - but if you optimized it down to just 4 moves plus 4 erasures next to the opponent - that might make for a quite manageable search space for use in a min-max tree). I've been playing with developing my own min-max addon, but currently it suffers from it's API being a little arduous. You can check it out here: https://github.com/Habidakus/min-max-godot-addon/tree/main/addons/minmaxcalculat...
I'm considering terming all my uses to AO (artificial opponent) instead of AI, since AI is such a poison word now-a-days since everyone associates it with LLMs, but using home grown heuristics and algorithms is half the fun of these sorts games.