as far as I’m aware the 752 National is the launch aircraft for BOS
No, there's been some further changes...
The A321neo now doing STN-EWR daily will reduce to 3x weekly for a brief period to allow for the 4x weekly BOS launch on 18th May. The second A321neo arrives on 30th May from which it will take on BOS and the YYZ launch. The first aircraft will then go back to doing EWR daily. A third A321neo is still set for the IAD launch on 22nd August from which time YYZ will increase to 5x weekly.
The National 752 it seems is now being based in BHX until the 29th July. It will operate EWR now only 6x weekly until 22nd June, from which time it'll reduce to 3x weekly to allow for the 3x weekly YYZ launch. EWR will then go up to 4x weekly once the A321neo takes over.
To confirm, the National 752 will only be utilised 6x a week, at BHX, from 18th May to 29th July. STN long-haul should now be fully serviced by the A321neo.