Scotland isn't my specialty, but I strongly suspect that Highland targes derived from targes and targets that were in use in England and elsewhere in the 16th century. Many parts of Europe used sword-and-shield men in various ways. Presumably Scotland just hung on to the fashion after it was outdated elsewhere, not an unknown thing in Scotland. So while there were probably "Lowland targes" in the 16th century, I doubt you'd see an equivalent in the 18th. I'd be a little leery of terms like "phased out", since that implies a deliberate action when we could simply be discussing shifts in fashion.
Oh, and tribal people at Mons Graupius were not Picts but Caledonians, and they did not have 2-handed swords. I only know of one medieval Pictish stone showing a battle scene, and one of the warriors is using a spear 2-handed, but it doesn't tell us much about formations, etc.
Matthew