> I guess this would always work with tally marks.

This script overlays the marks, it doesn’t put them side by side. So, in the strict sense, this does not work with tally marks. If you write a tally mark on top of another tally mark, you won’t get two tally marks.

> Is there a more complex number system where visual feedback like this always works?

No. The “+1” operator would have to add the encoding for the number one to whatever number you apply it to, so starting at 1, the numbers would have to grow larger every time.

(Similarly, since “+2” must be identical to applying “+1” twice, it must add whatever “+1” adds twice, “+3” must do that thrice, etc)