I thought you needed the planning at a fair price? And now you say you only take over the planning and construction management?
Basically, an architect charges according to a fee table. It depends on how complex your volume is, how much it costs in the end, and which planning phases the architect takes on. But actually, all architects should cost more or less the same.
With an architect, you are completely flexible regarding the execution. Whether your house should be made of wood, stone, metal, or whatever.
With BEWA, you are bound to their construction system, and the awarding of individual trades, construction supervision, etc. is missing. That means either you take over that yourselves if you dare, or you have to go back to the architect anyway.
So, with the 15K from BEWA, only part of the architect's services is included.
Maybe the architect can also build with the BEWA system??