Until this feature is released, here is how I would do it:
- Let’s imagine we have 50 groups of 3 columns each, grouped left to right from 1 to 50.
- I want to move columns 50a, 50b and 50c between groups 1 and 2 (therefore between 1c and 2a).
- I would rename Group2/2a to Group50/2a.
- Upper right in Search, I would type Group50: I should see Group50/2a, Group50/50a, Group50/50b and Group50/50c in this order.
- Now starting from the right, I’ll drag 50c to the left of 2a, 50b to the left of 50c, and 50a to the left of 50b. Now columns 50a/b/c are to the left of 2a.
- Rename Group50/2a back to Group2/2a.
- Click on the x in the Search to remove the column filter.
The explanation is probably more complicated than it actually is. The idea is simply to filter the columns to a few which makes the drag-and-drop process easier.
Column grouping in Glide Tables
(I do the xyz trick sometimes, same idea.)