Drag-and-dropping a folder on another folder in Speed Dial should combine the two folders, so that the contents of the first folder appends to the contents of the second folder.
Duplicate cards should also be deleted. So,
folder1
Card to google.com
Card to gmail.com
+
folder2
Card to google.com
Card to youtube.com
ā
folder1
Card to google.com
Card to gmail.com
Card to youtube.com
I like to use the "Save tabs as Speed Dial folder" feature when I want to switch gears and work on a different set of tabs, but then I end up with multiple Speed Dial folders about the same topic, and it's really annoying to drag each card from inside one folder into another.
I do know that Workspaces exist, but I'm not sure of the resource consumption of those things and my laptop is a potato so I like to just close tabs I'm not actively using.