Great post - I didn't realise there was so much left of the Antonine Wall !
It was really strange that the Romans designed buildings to last for centuries, but when it came to expanding the Empire it was entirely down to short-term expedient.
It would have been eminently possible for them to pacify the whole of the British Isles. The tribes were fierce, but the population density in Hibernia and Caledonia was much lower than in more southern parts. The policy of Vastatio (total scorched-earth devastation) while harsh, would have been effective against those tribes who didn't accept trade, client kingdom status and the increasing Romanisation that went with it.
However, it would have taken decades to achieve this, and most emperors wanted a quick victory rather than to spend time developing an outback region. But if one of them had done it before the start of the third century, it could have enabled Britannia to be reduced from a 3/4 legion province to 2 or even 1, and the new tribes would have been a valuable source of additional auxilia for the Danube frontier (75% of British Auxilia units appear to have been stationed in Pannonia and Dacia).