Being someone with poor handwriting, this would slow down writing for me. A possible answer for this would be to update the expected places for a character by taking the boundaries of previously written ones.
Writing in monospace is not only unnatural, but the approach shown even makes it one step worse: you need to write in the cells. You're completely giving up the flexibility of handwriting, you need to know your equation before you write it down, otherwise you're risking running out of "cells" on the left or right.
The problem you mentioned – that you have to plan ahead – is not true for grid cells. There is a part in the GRAIL system demo where it is shown that you can go quite far with this approach. [0]
But as you mentioned, flexibility of handwriting is in its non-modal interactions. I am currently working on a line-based editing, but it's a bit tricky.
Historically, graffiti-like systems have also been used for "night writing", so they might have some accessibility benefits as well.