The research indicates that the lower traps(T6-T12) are a phasic muscle(weaker), what exercises target this area?

Yes better yet is standing with palms up and pulling from above with elbows bent at 90 degrees down to chest. Squeezing the lower traps at end range.

The best is prone with your palms down, arm abducted slightly(~30 degrees) and raising from floor to bench level

Low traps are not worked enough and properly!