First of all, I am extremly humbled that you chose to spend that much time with Core Contest. Luck of the draw is an important factor in this game, especially since due to time constraint the rewards for different playstyles are not equal. Focusing more on puzzle element is an interesting idea that I will give more thought to.
The game is difficult in its current form, and the difficulty is also uninetionally amplified by the lack of proper tutorial, but it should be beatable*. If you have more thoughts about the game, balance, or whatever else I would love to hear them as you are probably the person with second most experience (I am on ShovelJam discord under the same nickname).
*There may be some extreme circumstances that I'm not aware of, but having completed couple of runs without random spawns to test this issue it can be beaten, but I am a dev so I'm basically cheating.
I couldn't manage to beat it at all no matter what strategies I used. By the end I just didn't have enough space left to put down the higher point value blocks in the arrangement they needed. I couldn't find any way to delete blocks I'd already placed. So I struggled to get to 500 and could never reach 700. I hope that feedback can help! - Bonnford