On the Proton Mail inbox (https://mail.proton.me/u/5/inbox) scroll down to 'Folders':

Add a sort button right next to the + (add folder) button and before the gear (settings) button. This would eliminate the need to load another page to sort the folders and then reload the inbox page.

Also, as others have recommended, allow the subfolders to be sorted alphabetically.

Optionally, allow the folders to be sorted when they are created