I've taken twelve thousand pictures using LR Mobile. Mobile has also imported many pictures from Photos. They have synced to my desktop via the CC Cloud.
I have renamed, added keywords, uploaded to Flickr, et cetera, mostly using the desktop app. Now I want to take 5,000 off my iPhone but keep them in my desktop library. If I delete the picture from the Camera Roll it gets deleted on the desktop. I'd like to recover several GB on my iPhone.
e.g. Some of the pictures are receipts, useful at tax time, don't need to carry them around forever on my iPhone and iPad; some are medical pictures I'd also prefer not to have on my mobile device; I have vacation pictures from 2016 I'd like to archive on my desktop.... can't figure out how to get them off the iPhone and iPad without deleting them from the Library.