I have a bucket but it was used as phone tripod
Regarding upper body essentially keep upper body vertical (side to side direction - of course there should be some forward lean) and just use knee and hip flexion to get down?
it seems some pros have some side bend like Dima sometimes but others are more upright
Some of these things are the reasons why people get back injuries, despite their fitness level (Dima and Timo and even Zhang Jike are not exceptions and I am also speaking from personal experience). You have to be careful with what you copy from people. Carl has explained to you that it is very different to drop the body with a lunge and then play this shot (which is still dangerous even then to the back) and to make it a part of your base stroke. It might feel good for a while but when the discs slip, unlike those guys, you aren't getting paid millions to compensate for the issues that follow.
One of the things people miss is that a lot of pros play even their base topspins with sidespin so that they don't have to drop the racket so much - they get more forward power and kick. Here is one of my favorite examples here. He clearly has one of the best forehands in the world and you aren't doing what he is doing.
The way I try to teach the forehand topspin, rightly or wrongly I really don't care that much anymore, is to encourage people to expose as much paddle to the ball with their hitting technique. Then you adjust from there by going through the ball with maximum surface area exposed while closing the paddle a bit more and brushing it slightly more - it adds some sidespin or corkscrew, but you can fix that with minor adjustments. Not saying I am right or wrong, and sometimes we project too much, but you are trying to do a lot to look like a good player, but if you watch the good players, they make good contact with the ball. They only brush really heavy backspin balls and that is when they are out of position. On many of the balls you are lifting, you should really be able to play through the ball with spin. If you want to drop your paddle, you need to work on your legs and get your body low and play the same stroke that you played when you were high, not ruin your back. When you ruin your back, you aren't making the money they make for doing so. Always remember that.