OP I don't think you need the discovery of the gold veins for Rome to decide that Caledonia et Pictonia is worth the effort as Romes capacity to discover it is very limited seeing as the Scots only found it a few decades ago. Instead, the Scottish lowlands had a lot of lead and Rome could of held it for this alone, similar to the rugged Britannia Occidentalis. To acomplish this Agricola is your man, and only after a few generations of a Roman hold on Britannia, Caledonia and Hibernia is enough to prevent the construction of a static border in the form of Hadrians and Antonines walls, though I still think that very marginal regions would be not held by Rome so the highlands and Connaught. There were other regions Rome could have conquered for mineral resources such as the Ore mountains that were not conquered so Rome was not poor of options if it decided to expand its mineral production capacity. Roman production of mineral resources declined following the second century but even still the Romans were very resource rich, see this wikipedia article for instance.
en.wikipedia.org
None of this will have a significant impact on preventing a fall of Rome similar to the fifth century as a firmer hold on Britannia would not deal with the political, social and corruption issues of the late empire. As for romanistation, it would leave Scotland as a Celtic and not Germanic nation (I know people think its celtic but imo its not) in the alternate timeline, with a Romanisation similar to Roman Dumnonia and Wales. There may not be a Scotland, instead Scottish medieval history would probably be similar to that of Wales in our timeline.
Map of non ferrous metals plus iron. Sloppily highlighted land is area that I think Rome would take.