Originally Posted by
trailangel
5'9" with long legs on a 54cm?
I think you are going in the wrong direction.
Put the saddle all the way back.
I think I'd disagree with that - 5'9" with long legs means he has a short torso. The reach is too long and it's causing shoulder and neck issues.
I'd definitely try moving the seat forward, maybe checking the tilt (if you're tilted forward you will have more pressure on your hands/arms/shoulders). I tilt my seat on my road and gravel bikes slightly back. Then possibly put a spacer under the stem to raise the bars just a bit (use a 4 or 5mm spacer). Depending on how the steerer tube is cut you may not be able to put a spacer under the stem - if you have a couple spacers above the stem already just move one under.
You said you're more comfortable on your trekking bike with a more upright position...the changes above will make you more upright.