How do a make a new one of these? I know I can edit one, open one preset > edit > save them as startup files. But how do I add say a 6th preset…
Items of New File menu are Application Templates.
You basically just save startup.blend files with desired workspaces, in folder named as template.
Then, you just have to move this folder in ./config folder.
Blender icon is a clickable menu, that allows to install application template from shared archive.
Additionally a (maybe) slighly simpler step before going into “template production” 
.
One can “simply” copy any workbench, adjust and rename to one needs and then only save this as startup file. After restarting blender this new workbench is part of the “new working suit”.. (with the same method one can “delete” a workspace from the default setup..)