Welcome to the Cesium community @jonas.rodningen!
I believe CesiumJS already applies this glPolygonOffset fix when the browser supports it (according to the PR here https://github.com/CesiumGS/cesium/pull/4559). I don’t know that there’s an easy fix the application can do to improve these artifacts on shadows other than increase the size of the shadow map perhaps.
@sean_lilley is this something that would require a different approach to shadows in the engine, or something we should have a GitHub issue for? This also came up again here: How to use glPolygonOffset inside fragmentShader to reduce z-fighting