"Gorilla arm" means that holding up your arm(s) without rest will eventually make them achy and tired, even if you're reading a single sheet of paper.

Standing desks are great, for those who can personally afford them, or can obtain employer accommodation.

For exercises to improve posture and relieve pain, I've been relying on this YouTube channel: https://www.youtube.com/@posturemakeover7165 She's selling a 30-day program, but the introductory material is comprehensive and life-changing.