“Who were the Picts”
That’s a great question. Some think they were Celts, maybe, maybe not. The Celts were from central europe and spoke an Indo-European language. (The Iberians weren’t Celts ) Early missionaries considered their language very different. Venerable Bede called the Picts, “Scythian” which some think means Scandinavian. What if they were exactly what he called them Scythians which were a horse culture from the Eurasian steppes and Black Sea. The Picts had horses and horse culture. I can’t help but notice how similar images of Pictish horses are to Scythian. How they got there is anybodies guess but it’s not to much of a stretch to see them as mercenary horse soldiers stranded in a strange land who got the urge to settle down.
I don’t know why you say the Iberians weren’t celt.
Late ancient authors, dealing with various ethnic groups not mentioned in earlier historians, often applied to them the names of unrelated people who had lived in the same area many centuries earlier who were mentioned by Herodotus and other early writers. So tribes living north of the Black Sea might be called Scythians even if not really Scythian. It may be that Bede was following that practice (although the original Scyths were never as far west as Britain).