Folders seem to always come first.
Is there any way to make Folders also sort mixed in with Galleries, all by Last Modified Time? Folder's modified time would be based on the galleries within it.
The only way I am aware of to do this is to use a Folders, Galleries, and Pages content block and select the items manually.
Unfortunately there isn't a way to specify a sort sequence and in my experience if you choose to add new items they will (usually) get added to the end. The only way to reorder is to reselect all of the entries.
You can remove items back to where the order is different and add new ones.
I open the page in a new tab so I have a reference of all and the old order.