who were stationed on Hadrian's wall, having been forced to serve Rome as a legacy of a war between the Sarmatians and the Romans (from quick wiki searches, I'm guessing the Marcomannic Wars[2] - that's a random guess though)

Correct. Cassius Dio reports that 5,500 out of 8,000 Samartians captured (in 175 CE) were sent there (Dio, 72.16.2); and Marcus Aurelius assumed the victory title Sarmaticus.

I don't know if you saw it, but I had asked a related question about Sarmatian connections here. There weren't a whole lot of responses, but /u/Whoosier commented that

the basic problem with the Sarmatian theory, which is that most of its main elements enter the Arthurian legend only after Geoffrey of Monmouth's first mythic elaboration of the story in his 12th-centiry History of the Kings of Britain. This presumes a thousand-year interim between Lucius Artorius Castus and the commemoration of his deeds

We should certainly bear these previous cautions in mind; but I can't help but note that at least the name of Arthur speaks for a long vitality of tradition. It speaks to a historically attested Roman nomen/gens (Griffen 1994) - with the historically attested commander Lucius Artorius Castus, of this gens, being stationed in Britain in the late 2nd/early 3rd century CE.

Whatever possible Sarmatian connections in the Arthurian literature itself will be judged on their own merits (I have no stake in the debate; though I do note that the reviews - appearing in the journal Arthuriana - of a major work that proposed a Sarmatian connection were not immediately dismissive...one noting that certain evidence was "highly suggestive of a connection between Scythian and Arthurian legend"); but it's not as if people are proposing that the Arthurian tales emerged in full form in the 3rd century, but weren't written down until the late medieval period. If there was a thriving Sarmatian community, we would expect a vitality of tradition throughout the centuries. Hell, tales from early Mesopotamian literature are still preserved in the oral tradition of modern Iraqis.

Almost all of the mythology surrounding King Arthur is fantasy. In fact, the only historical thing we can say with relative confidence regarding the tales of King Arthur, is that the Battle of Badon Hill was fought, but not that Arthur was there. This would be from the only surviving document we have from around that time, by the cleric Gildas, who was born in the year of the battle.

After that we have the Historia Brittonum by Nennius (maybe) which is at best dated to 300 years afterwards, and the Annales Cambriae 100 years (maybe) after Nennius, both only mentioning Arthur's existence and participation in particular battles.

Much of the framework for the legends as we know it come from retellings hundreds of years after his supposed existence, from either Geoffrey of Monmouth, Chretian de Troyes, or even Malory himself.

So given these caveats, its probably no surprise that any scholarly historian (as opposed to popular ones) sets aside Arthur in any discussion of the era.

As Guy Halsall pointed out in his recent book on the subject, Arthur may have actually existed, but the almost complete lack of documentary evidence means there is nothing we can usefully say about him historically.

The portrayal of the Saxons in that movie was, from top to bottom, wrong in nearly every way in which it is possible to be. The Saxons didn't invade in one fell swoop; they came in drips and drabs, perhaps initially brought over as mercenaries by the Romano-British. They certainly didn't land in Scotland and come down through Hadrian's Wall; they landed in the south and southeast, the so-called Saxon Shore. The Saxons weren't a united and monolithic entity; as late as the 7th century they were divided into several petty kingdoms. Especially in the dynamic of the war leader's relationship with his followers they completely miss the mark. A Germanic war leader was not generally able to rule autocratically. He maintained control by placating his followers with loot, listening to their advice, and rewarding them with gifts.