Drywall/Brick Transition - how to paint?
I think you'd have to support the drywall to make sure it does not flex. That would involve demolition, adding support that is evidently lacking now, and replacement; and may not be worth the effort.
Placing a molding over the crack to conceal it would be another approach.
Cutting it out to 1/4 inch wide and filling it with caulking as an expansion joint is a third option, but probably less appealing. You'd likely want to hide it, in which case you dont need to bother, you can hide the crack with molding, as mentioned just above.