I don't know much about this stuff but I'd imagine there's a way to make a shortcut activate sidecar for ipad with a keyboard combo or trackpad gesture.
Can't be done from preview and would be nice to as well but I think that's a long shot.
It requires: from finder, go to the file, right click, quick actions, markup, then click on annotate on iOS device.
I'd like to do this in a simpler manner, any advice?