Over time I grew to understand that different adjustments (saddle, handlebars, pedals, stem) affect different parts of the body. Often multiple at ones and often indirectly - by altering something else directly and then for example putting more strain on other parts of body indirectly because of that. My girlfriend recently started riding a road type bike (gravel) with flat pedals. Her neck hurts. I think my neck hurt when I became a roadie as well. But that went away after a few months. I'm not sure if I accidentally fixed my position then or if my neck muscles/whatever is there adapted.
I have tried changing her handlebar tilt upwards, and ever so slightly (1.5mm) moved the saddle to the front. I'm pretty sure there is nothing extreme about her position or the bike setting. The frame size is definitely good. I have also suggested that she moves her body around on the bike: stand from time to time, move the neck every few minutes, change arm position on the handlebars.
Does anyone have a good advice which parts of the bike can fiddled with to experiment and try to fix the neck pain?