Since I need to control exactly how large the offset is to fit the logic of my custom shader I solved it now another way: Per mesh projection matrix
The problem with the polygonOffset() would be that I would need to know the value of “r” which is described in the docs as r is the smallest value that is guaranteed to produce a resolvable offset for a given implementation. Might be that one can somehow query it but like I said, I solved it by modifying the projection matrix.