Thanks for this. I created 2 shortcuts. 1 to combine to PDF and save into the parent directory of original files, and one to split a PDF into multiple pages into the parent directory or original PDF. Both accessible from Quick Actions. Works great, only downside is Shortcuts asks for permission for parent directory if it hasn't received authorization for that folder before. For Desktop, I imagine this happens once, but it could get a little annoying. The alert requesting permission does respond to a press of the return key though.
You can add these to your system by clicking the links below.
Combine to PDF
https://www.icloud.com/shortcuts/3318ae68edaa48c4b434938de48866bb
Split PDF into Pages
https://www.icloud.com/shortcuts/1e3e624a20ab42f09da635412c93bacf