I have continued to optimize; corrected a small error I made that caused the walls for the title and puzzle screens to not display correctly; and have been testing on real hardware.  The game works well on the Game Boy DMG, Pocket, Super, and Color.  I noticed on the Color, the start button was a bit squishy and it would trigger multiple times when pressed (this might be a hardware issue with just my Color).  This sometimes interfered with pausing and unpausing the game.  I corrected this by increasing the required delay between pressing start before the game will register it again.  After observing some other people playing, I also lowered the game speed in the campaign for the first two levels so new players can learn, but the difficulty of the last three levels is unchanged.

I won't publish a new version just yet because the above changes are very minor.  But once I am confident that there are absolutely no other bugs or issues that require correction, I will publish the final release.

The test cart I have been using was ordered from Geek Tech on Etsy.  It arrived quickly and was very good quality.  If anyone is looking for a shop to create homebrew cartridges, I recommend them.