Welp...I think my hand is forced to go back to my original option - rebuilding the "columns" property each time new data is retrieved.
Haven't found anything to center the header text and set number formatting without using the "columns" property. This is what I tried in a scrip transform on the data binding (was really crossing my fingers).
Editable with UID - works well, "justify" is unnecessary
Tried some formatting with no luck (other than the align)
I think I'll still stick with using a flex repeater for each group as that is working out well. Had to build a list of objects for each row since the data is transposed and some cells are editable so I had to include a UID value for each cell in order to edit (first picture).