What are some key road flat pedal bicycle adjustments that affect neck strain?
If a rider is suffering neck pain, one of the first things to try is removing any visor from the helmet.  MTB helmets frequently come with some kind of sunshade/peak/bill horizontally over the eyes, and when on a road style bike, that means craning the neck up more to see underneat.  So unclip it.
Rider can also try to ride with a straighter back.  Its easy to hunch over so one's spine is curved.  That also puts strain on the neck.  At least try and straighten the back periodically while riding.
After that, it becomes a bike fit process, which is getting a bit much for an answer here.