@Dragy , can we say that I let go of my left hand too early/too high because I start the forward swing too high? Or is it just because I don't initiate the forward swing with torso rotation but mainly with my right arm?
I see it as timing issue. By setting your arms so high you make your swing longer - cannot just skip the drop part by rushing it. It's kind of like DelPo FH when he's not rushed - you see it starting slowly, but you know thunder to come inevitably, just a moment later. Compared to Rafa FH, which kind of happens much more rapidly. Imagine trying that Rafa explosive thing, from DelPo high takeback - well, a flat slap maybe?
The thing is, all best OHBHs are more like DelPo. Yes Fed pulls off some super-compact ones, and Wawrinka is middle-of-the-road, producing great RHS relatively compact. But still, he loves to hang back, where he has time to prepare and roll-start the swing.
So if you want to develop further your OHBH drive, I'd attack it from timing standpoint. It's tough mentally, but play with uncoiling as the ball approaches - not at the instance you need to hit it. You may want to try it with racquet taken directly to "slot", and of course it's easier. Just be ok with softish shots as you practice, hard rips will come later on.
Look, you actually want to swing at the ball from here:
You can use both hands to directly put it there:
But to get to this point with good energy input you do everything else. You coil, prep, step, drop-start your swing. So that buy the time you split hands it's already moving and naturally takes off to "high orbit" outward.