I have a folder on the laptop which basically duplicates the camera folder /storage/emulated/0/DCIM on my android phone.
I organise the photos on my laptop into folders (normally based on events) and those subfolder get synced back to the phone. If I want to delete photos on the phone, that I want to keep, I just move them from the synced camera folder to another (not-synced) folder on my laptop. Photos are then still on the laptop, but on the phone they are gone.