I am using the Galaxy S7 Edge, and have no real issues, remember the camera is not 4K, and I doubt the signal being transmitted to the phone is at full resolution. If the full resolution is being sent to the phone you would loose out on signal strength, I believe this is why DJI has a different video transmission system on their better drones to save resources for the control system.
Which mobile phone will give me the best quality of view when using Cockpitglasses FPV?
I used Galaxy S7 but view was not so sharp with pixels visible.
Should we wait for some 4K screens?
I think this is more signal issue,good wifi signal without interferance should give you a good fpv video,....making sure that phone doesnt have to many unnecessary apps running in background might help too