Clem von Jones

Try lowering the bars first. Get them as low as possible and try that out. Flip your stem and remove all spacers. Lowering the bars will reduce the fatiguing jackhammer effect that travels up your arms into your shoulders, neck, and upper back. The lower the bars are the better it will feel to have your saddle in a slightly forward and your reach long. If you do the opposite and raise your bars you'll have to use a much shorter stem and move your saddle backwards. Doing that will very likely have no positive effect on your back fatigue. I know that sounds counter intuitive but that's how it is. Experiment and see for yourself. Your spine is like a suspension bridge that will support your weight if you can get the bars low enough. At some point, if you keep lowering the bars, most of the weight will come off your arms and your hands will rest lightly on the bars. That's ideal. Having more weight on the front wheel also makes the bike handle much better.