We can set the command order in the ribbon by using the Sub-menu name and the Sort priority in the command configuration.

The Sub-menu seams to appears based on the order the command were created.

If want to create a new sub-menu and have new commands to appear at the beginning, the only way to make this work is to delete all commands and recreate them all in the desired order.

Can you have the sub menus to consider the Sort priority too?