I would definitely recommend a sit-stand desk. Variation is key. Try to figure out if holding your shoulders up or down makes things worse and adjust your arm support accordingly. A good desk chair can help, with adjustable or removable arm support. You can try out different ergonomic keyboards or mice, but that can get pretty expensive, with no guaranteed results (and lots of frustration). Look on eBay for used ones or find a vendor with an extended return policy.

Take enough breaks, no matter what. Take a walk, no matter how short. Get the blood and nerve signals in your arms flowing.

Switch between desktop, laptop or tablet during the day. Use them for specific tasks (like reading mails on your tablet, development or content creation on your desktop and casual browsing on your laptop.

But I’m pretty sure you’re already familiar with most of my advice.