deleting 'some' synced photo

Hello. I just came back from a trip, and I have 2000+ photos that I took with a different device, that I have to deal with in terms of what to save and what to delete. I'm trying to sync all 2000+ photos into my iphone and do the save/delete process in the phone, rather than on the computer, because it's too slow. But I know that I'm not able to delete those synced photos on the device. What are the options to go about doing this on my phone?

I have tried 1 option so far: go to that synced photo album on my phone with (from my computer) marking. Selected all photos, added them and moved to a new folder with a new name. From that new folder, I can delete selected photos, but the number of photos from that (from my computer) folder remains the same. Tried removing the (from my computer) folder by unsyncing from iTunes, but the photos in that new-added folder also deletes together.

Plz help 😟