I have a book about Sarmatians and there is reference to archeological finds in the Black Sea area of the Ukraine, Porogi, Sokolova Mogila, near Nickolaev, Ukraine, and Azov. These relate to quite high ranking Sarmatians or Alans. It does place them without doubt in the Ukraine and given that they were nomadic there would be movement.
So there could have been several waves of migration but I guess that the time zone you are looking at is after the defeat of the Romans when the Alans seemed to go off in all directions.
One point I would make which may seem a little strange. There was a comment on a web site about the legacies that the Sarmatians that came to the North West area of England had left. There was a remark that the cattle in the area were descended from the Poldonian Grey Steppe breed introduced by the Sarmatians.
Apparently the Sarmatians introduced this breed of cattle into a number of areas.
Regarding the Alans in France, a British tribe from the South West of England were driven out by the Saxons. They settled in Brittany and intermarried with the Alans that settled in the same area and became known as the Bretons.
In 1066 William, Duke of Normandy laid claim to the English throne and Fought Harold Goodwin at Hastings. William conquered England and the Dukes of Brittany
(Alan decendants) fought in the battle. These Alan descendants would have been given lands in England and would have been powerful lords.
So there were two waves of Sarmatians/Alans in England which may have both given rise to the Arthurian legend.
In the bibliography of The Sarmatians 600 BC - AD 450 there is reference to a number of ancient works which contain significant references to the Sarmatians.
Ammianus Marcellinus AD c330- c395 Rerum Gestarum ('Histories')
Vegetius (Written c390 AD) Epitoma Rei Militaris. ('Epitome of Military Science') Translated L P Milner 1993.
All other primary sources are much earlier than your time scale.
Bernard S. Bachrach, A history of Alans in the West, Minneapolis 1973
Bernard S. Bachrach, Armies and Politics in the Medieval West, London 1993 (collection of essays)
J. Harmatta, Studies on the History and language of the Sarmatians, Sezged 1970
V. Kouznetsov and I. Lebedynsky, Les Alains, cavaliers des Steppes, seigneurs du Caucase, Paris 1997
D. Sinor (ed.) The Cambridge History of Inner Asia 1990