It has been a long time since I played, so trying to figure stuff out. I had been hitting with a bent/bent which I don't think is good for me as I don't hit out in front and break the elbows too soon. I'm going to try to go with bent/straight or straight/straight but really focus on that left arm being straight. It was amazing how much further out in front I hit the ball, more easy topspin and control just focusing on trying to hyperextend left arm. "Hyperextend" Well that is what I had to tell my brain to make it stay straight. When I tell brain keep arm straight and think that. It's not until I look at video my arm was still always bent and bent by a lot. Werid. I have to think/feel hyperextend.
I have gone to a couple of private lessons but not sure who is the right fit. I'd really like to find someone super technical like what everyone on the thread is saying. My backhand has improved more in one day posting here and trying to replicate the advice on video than all my recent private lessons. Granted they weren't solely focused on BH. My #1 issue is spacing I try to get too close to the ball.
Welcome back to tennis ... have fun.
A couple of things on straight left arm ... that is a "at contact" definition. If you watch Cilic or Zverev for example, they have bent arms in unit turn, and left arm extends to straight by contact. I came to the conclusion everything before the slot is just preference and prep ... too many pro varieties to say it has to be one way. But from slot to contact ... pros start to look more similar to each other.
Most pro 2hbh hit a little in front of front foot on closed 2hbh, but not typically way out in front like their FHs. "Extension" generally refers to extending non dom arm forward toward net "after" contact. I have several pics on imgur of pros ... will add post when at the PC. The reach and spacing changes for sure with 2hbh type ... big reach difference between a Venus bent/bent and an Agassi straight/straight. Not as big a difference between straight/straight and bent/straight. But again ... that is dictated by arm position at contact, straight arms in backswing just prep.
Do you hit open/semi open 2hbhs yet. I waited to add it to much later ... on purpose ... because my goal was to nail down one good quality low ue stance first. Now that I have added semi open, I think I should have introducec it much sooner. For one thing, you need the option on groundstrokes and ros. The other thing is the technique above the waist is the same across strokes, so reinforces the good and points out the bad.
Have fun ... look forward to your updates.
Edit: generally, when you hear 2hbh extension ... this is what I think it refers to. I'm at the point where I do it when I think about it, but I think I fall back into the habit of the elbow folding too early on follow through too often.
Edit2: I have never tried using the medicine ball to ingrain a stroke, but the coaches here (
@J011yroger ) are big proponents. My guess is if you tried to throw a bh medicine ball off your right leg, the weight of it would force you into the habit of being all the way on that right leg before you could toss the medicine ball with any enthusiasm.

Jolly ... is that part of the medicine ball idea ... nail down proper stances ... besides any other strength things you are after?