I designed a similar aircraft for dropping milk jugs full of water. I installed the stab 3" below the wing and never had any problem with pitch control, it did pitch up slightly with rudder. I flew as slow as possible when loaded to avoid pulling Gs. The aircraft trimmed to speed well when the cg was 15% mac. Flaps may be overkill, I used full span flap-flaperons and and they were fun to play with but not much value. It landed slow without them and really slow when deployed, aileron authority was almost non existent. I think the biggest asset to high alpha control is in keeping the cg forward .