There were a couple of things that really helped me - some you're already doing.
- I used to use an E. fh grip on my left hand. I went to a extreme E, almost SW grip like you are now.
- I allowed my left hand to control the racquet. I really think of my 2hbh as a left handed fh now. I used to think of the hands more equally controlling the racquet, but the effect of that was that I had no looseness in my wrist, therefore no stretch-shortening to whip the racquet. My right hand grip on the racquet is very loose during take back. I tighten it up just right before contact to help whip the racquet forward.
- I have become more mindful of the contact point - especially allowing the ball to come all the way to me instead of pulling the trigger too soon and taking it too far out front. When I take it too far out front I loose the topspin.
- In my take back I try to keep the head up during prep, then allow the the head to dip below the ball and whip it forward and up. I can control (most of the time) how much topspin I get by how low below the ball the head dips. When I let the head drop during set-up I loose power and spin.
- I stopped worrying about whether my arms were straight or bent. I always kind of thought of my two hander as both arms straight at contact (and certainly there are players where that's true), however after filming myself and noting when things felt right, for me the best backhands where when my left arm was mostly straight and my right was a bit more bent.
With this all said, I still can't generate the same amount of topspin off my backhand as I can my forehand. But I can get good pace and margin over the net when I hit it right, and on good days I can push good players back off the baseline with pace.
What are you experiencing with your bh? When does it feel right? What parts of it work for you right now?