If you haven't tried this already: In Lightroom Classic, Library module, look in the Catalog panel. Under the All Photographs collection that you already looked at, there should be a collection called All Synced Photographs. If it still lists many images, those are the ones still synced to the cloud and therefore listed in your iPhone. Select All Synced Photographs, then select all of those, then press Delete. You should get a warning that they'll be deleted from the cloud but not deleted from the desktop catalog. Confirm that.
Then switch to the iPhone and see if the images are still there. They should be gone after the next automatic sync.
Thanks Conrad. I'll try to experiment with deleting the synced files. So far I tried to move 3 GB but the space in LR mobile increased only by 400 MB. Maybe it needs more time than 15 minutes to detect the changes... everything was synced and looked fine (the number of all synced photos decresed) but I expected to see bigger change in free space.
Btw is there an easy way how to see what files are stored locally on the phone, and what are in cloud?
How can I find out what's the real size of 'All Synced Photographs'? I don't understand it because Cloud Storage & Sync shows me 13000 photos and 94 out of 100 GB available cloud storage (that equals to 6 GB used), however in Local Storage section I see 38 GB under locally stored copies.
I tried to browse all synced photos in Airplane mode to see files stored on the phone but I see thumbnails only of few collections which I chose to store locally (4 collections with less than 300 photos)... nothing else... and that's weird because they definitely can't take up all 38 GB.