I don’t know. I think the current Shield Expert perk isn’t sufficiently attractive, since the defence bonus is too meager and the only real point of it is to prevent orcs from smashing your shield (or at least to force them to exhaust themselves in the process of doing so). It would kind of make sense for shield expertise (like weapon expertise) to reduce the fatigue costs of using the given tool, and also to increase the benefit of certain skills. But maybe shield Knock Backs shouldn’t impose damage and/or fatigue. Or maybe shield skills could be made more specific, so that (a) small bucklers can’t be used for Knock Back at all but can be used for a punch-style attack that does modest damage, and (b) larger shields can be used for Knock Back but not for punch-style attacks that do damage.