I'm pretty sure Swiftstrike is the most popular item without a 5* version of itself right now. I suppose adding a 5* Spur was more important on the grounds that weapons make a bigger impact on gameplay than new shields do (people even using the SSB in the first place kinda proves this), but players need a 5* shield to progress through the hall of hero's quest, and the striker merchant is outright wearing a SSB despite it dead-ending at 3. That issue combine with the buckler's general popularity makes me think it should have a 5*.

I'm fine with it being a weak shield. I get that it being only 3* is sort of the gimmick. So, instead of adding much in the way of health or defense, just give it movement speed low at 4* and medium at 5*. This would help keep it from getting invalidated by UVs, since movement speed can't be rolled on anything or even capped at the moment. As it stands a swordie is always better off just rolling for ASI on their weapon and then going for the thorn shield's damage instead; extra movement speed would put Swiftstrike on the map as something besides just a discount UV.