Couldn't get to sleep, last night.
The Level Layout grid was keeping me awake, as I considered how many alternative ways I could figure out the many possible sets of numbers.

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In the end, I spent another 2 or 3 hours fiddling with numbers, trying to find quick ways to redistribute the values, but eventually gave up.

The spiral is good enough. Since the levels aren't necessarily in order, anyway, having the grid be slightly misaligned isn't too much of an issue.

In the future, if I ever do figure this out properly, I can easily update it as it's part of the main framework, not the individual game.
Whereas the last time I had a grid-based level select, that was hardcoded directly into the specific game. This time, being a reusable part of the framework probably means you'll be seeing more of the gridded level select in future.

So, back to the game then!
Today I need to add some SFX, and get the "Level Complete" to work, and then add a tutorial to the first level, which should help players figure out WTF they're supposed to do.
It's quite an easy game to understand, but like I've said, can be incredibly complicated to play, so the tutorial level will indeed be necessary.

After that, I *think* everything important is done.
.. Hopefully!