Knowing the exact mag alloy woud help.
Aluminum and mag really don't get along well when welding.There can be some serious metallurgical problems.
As far as I know-there are no RELIABLE buffer alloys.At least for welding.
My first thought (like yours) is to go to a hi mag content aluminum alloy.5356 and 5556 are the highest at 5.5% or thereabouts. No sense, usually, exceeding the alloy of the weaker material.
In this app, however, you are finding better results with a mag filler.
I would go to AZ61A, for it's ductility.
Watch your heat. Too much heat can sometimes look like impurities.
Mag can give the appearance of "boiling". If this happens, your heat is too high. Grind out as this will be a pourous weld. A couple of weld/grind cycles should give you the results you need.
Hope this helps
Good Luck
brande