I went through something similar a couple years ago; like you, I decided I needed a flatter swing plane (I don't really remember why), and my swing with longer clubs, especially the driver, looked a lot like yours. By taking the club back so flat (below your shoulders), it's very difficult to keep your right arm connected (as others said), and you're forced to pull the club around with your torso-- coming over the top is usually the result (always was with me). It may also make it more difficult to release the club in time, resulting in an open club face at impact (again, it did for me). What helped me was to actually go two-plane for a while-- I took the club away high, then let it "fall into the slot" to begin the downswing. I had success with it, and I guess my swing evolved from there. Eventually, I took my normal back swing and stopped at the top in front of a mirror, and what felt like a high takeaway was actually right on plane. Anyway, a good swing thought I picked up at the time was to "swing around your neck, not your body"-- that is to feel like your neck is in the middle of the "circle" traced by your club head
That may not be the most mechanically efficient or orthodox way, but it worked for me-- but then, I still only shoot in the mid-90s, so what do I know. I hope it helps!
Also, don't rush your downswing. I think that might be what the whole "rhythm" post was about.