To anyone coming here: the solution from mike works perfectly. Even updating in the visual studio Designer during designing. I created this account specifically to comment on this.

I compiled his GridHelper class into a separate assembly and referenced it in my main assembly. After adding the xmlns reference it works exactly as he described. Link to the specific comment with the GridHelper code:

https://stackoverflow.com/a/74534620/29094387

Make sure you define both the row and column just like mike's example. You can then use the standard Grid.RowSpan and Grid.ColumnSpan if needed.