I'm having the same trouble as I learn my new boat.
First off i find trouble reducing sail area because it unbalalnces the for/aft water balance.
I use the boom track to get the balance to give fairly neutral helm
then things improve a bit.
I'm not sure part rolled genoa's do very much, the air stream breaks away too easily. I've got more work to do there on trimming/ shaping the sails
to be fuller and smaller to be more forgiving with pitching upsetting the working angles all the while.
My book talks about un-bending the mast
to make the main fuller, and adjusting the genoa
tracks for/aft to allow more belly there. All very confusing untill it comes together, and very hard work when it doesn't. Mono's don't seem to have so much trouble because they don't veer about so much. Feels like inadequate rudder
area when it's all wrong. Horrible and worrying but I don't let on too much. I'll master it sooner or later. Or at least get to grips with it.
Incidentally a recent trip on a BB385 in f6 against strong tide we had the same trouble. Unable to hold a course into wind with constant veering into wind, then falling off thirty to forty degrees again. Slow trip in unpleasant seas. Mono instructor didn't resolve it either. Practice, try, practice. Just need lots of water space and energy.