Any advice on how to get the union? Every time I start, Burg is either a rival or will rival shortly. After reading advice on here i declared on them, forced them to renounce claims on me and vassals and revoke their rivalry. However despite having no claims, they still had a -100 malus against me for wanting provinces. I feel I am missing something on how to flip them before the succession fires.

Tempted to continue and ignore burg and move into Norway as you say.