You bought the standing desk. Six weeks later your shoulders still round forward and your neck still aches by mid-afternoon.
That is not a defective desk. It is the wrong tool for the problem.
Rounded shoulders are usually not a shoulder problem. The shoulder blade sits on the rib cage, so the position of your ribs determines the position of your shoulders. Sit for years with the rib cage depressed and the mid-back stiff, and the shoulder blades tip forward because the surface they sit on has changed shape. Standing up does not change the shape of that surface. You now hold the same posture, vertically.
What actually changes it
Thoracic extension. Your mid-back has to be able to move before your shoulders can sit anywhere else. I test this with you seated, arms crossed, asking for extension while I block the lumbar spine so it cannot cheat. Most people I assess have very little.
Then the muscles that hold the blades back have to be strong enough to matter. Not stretched, strengthened. The reason your chest stretches feel great and change nothing is that the tissue holding you forward is not short, it is unopposed.
“Since I started, my posture has improved and my weak muscles have been strengthened.”
A more useful desk change
Position matters less than variation. The best setup is the one you leave frequently. Two or three postural changes an hour beat any single position, sitting or standing. Set your monitor at eye height so your neck is not carrying your head forward, keep your elbows supported, and get up often enough that no tissue holds one length for hours.
The equipment is fine. The equipment just cannot restore range you no longer have.