You have to account for fat fingers on mobile. Twice have I pressed the 4th square instead of the 3rd, and I don't even have "fat" fingers. Just remove the ending squares if they are operators.
Example:
1 + 8 + 4 + 2 + 3 + 3 + 2 + 9
Here you could use the left side to do 1 + 3 vertically.
For a larger grid to prevent misclicks, try going to settings and changing the "grid size" setting to a larger option.
Hope this helps! Thanks for playing.
Only potential problem I can see is if the grid line is for example:
3 + 4 + 3
And they accidentally press "3 + 4 +"
Hard to tell if they meant to press 3+4 or 3+4+3
If they meant to press 3+4+3 and the program auto removes the ending operator and submits it as 3+4, and it's incorrect, that might be more upsetting than having to quickly re-tap the equation they meant to submit