This feels like exactly the reason I prefer using zsh's zargs command for these things. In part because you can use all the extended globbing functionality zsh provides. Something like the following [untested] rework of your pipeline:

    zargs -P $(nproc) -i ***(#i).(pdf|azw3) -- calibre-convert \{\} \{\}.epub

We're firing up nproc tasks, matching case-insensitive .pdf or and .azw3 at any depth, and then batch processing them. zargs is documented in zshcontrib(1).

It doesn't feel that different to your imagined syntax to me, quite whether that matches your mental model better is for you to decide though.