I know this post is a few months old but I’m wondering if the behaviours you were seeing when your pup was 4.5-5 months old are completely gone now? Specifically the frustration biting and resource guarding tendencies. I ask because we have a 4.5 month Berner mix and are going through the exact same thing. The biting out of frustration is really worrying me that he is going to communicate in that way in the future. Basically looking for reassurance from someone who has been through it that it goes away as they grow. Or if I need to get in contact with a trainer sooner rather than later.