If you use Keyboard Maestro, you might be able to use it to get this to work. I do something similar with the annotation toolbar in Preview. I use an “if-then” conditional action in Keyboard Maestro. When Preview opens, the macro looks for the menu item that activates the annotation toolbar. That menu item is only there when the toolbar is not visible. It then types the keystroke to activate the menu item if it is available.
Mine was already set to “1”. I tried to set it to “0” and even “2” to see what would happen, but Preview presents the same behaviour: Thumbnails for multipage PDF, no thumbnail for single page PDF.
The value reverts back to “1” when I close and open Preview again.
I have an old license of KM. It really never clicked for me.
I don’t have it installed right now. In fact, I don’t even know if its compatible with the current macOS
Thanks for the input though.
I think I’ll have to resign myself to use the keyboard shortcut to display the thumbnails, at least for now (⌥+⌘+2).