Thanks for the answer! Your mod is simply the most logical and, at the same time, does not look too cheaty. Moreover, it fits into the logic of Factorio almost perfectly!
I have no questions about the "excavation tiles" themselves as a way to implement markings for soil excavation. It's just that the idea of creating markings as objects seems illogical to me.
Unfortunately, I don't know how difficult it is to implement, but in terms of logic, placing "excavation tiles" like drawings, it seems to me, would be more interesting.
There is one more thing that would improve the visual effect - this is the display of worked out but not filled with water "excavation tiles" - in the form of depressions using rocks.
But I think that it will be too difficult to implement. Therefore, do not take the last idea too seriously.
Thanks!