Marc. Good points. My workflow is similar.
I’ve never bottomed this out, but if (say) a piano pdf is incorrectly read as having 4 staves on page x (usually caused by a crease on the original creating a false line), after correcting and re-reading that page, the exported file sometimes retains extra “phantom” staves for which Sibelius requires “instruments” to be specified… Have you encountered this?
Also Photoscore really struggles with complex tuplets - especially where multiple voices are involved. Often I find it simplest just to delete the difficult measures and re-enter them from scratch in Dorico!
Despite its limitations, I still find Photoscore immensely useful to transfer pdfs to Dorico.