I think you're wrong there to be honest.
You need to mix it up , changing direction, speed and spin. Not just hit everything as hard and flat as possible.
If they hit a high one to me - I need to respond with a high one. When I can smack it then I smack it.
Hitting everything the same would be pretty stupid.
I didn't say hit everything the same nor did I say hit everything as hard and flat
If they hit a "high one" to you, that doesn't mean try to attack the ball above your shoulders, that isn't your hitting zone. Just take a look, for example, at the Nadal forehand crosscourt to the Federer backhand forcing him to hit up-high since Federer is too stubborn to change his court positioning. Roger can't do jack shit with the backhand like that.
Look, you even said yourself that your backhand has not been performing as well. Just take a look at your forehand, you do a better job at playing the ball into your hitting zone. Sometimes you just need to take an extra step forward or back so that it is right in the hitting zone. No one has the same hitting zone, and some are higher/lower than others depending on the grip they use and how much wrist they use as well.
Again, just look at the footwork you use on your forehand and how you time the ball well into your hitting zone. Yes, you aren't going to always be hitting "comfortable shots" but if you are being fed the tennis balls you shouldn't have an issue playing everything into your hitting zone. Your technique isn't the issue, it is just getting those feet in position and deciding whether to take something earlier or stepping back and letting the ball come into your hitting zone.
Look at this video of Nadal. Notice his footwork to move foreward and backwards to make sure his backhand is as close to his hitting zone as the rally permits.