The diagnostic targets are of little use if you are shooting two handed. They were developed for bullseye shooters.
The "ball and dummy" drill is equally ineffective. When you fire a shot, you compensate for the recoil. How? By moving the muzzle down. So, what is going to happen when you fire a shot and the next round is a dummy? The muzzle is going to go down. That is not a bad thing. What is a bad thing is when the timing is off - the muzzle dips before the hammer falls. All the "ball and dummy" in the world can't diagnose nor help that. I know for some this is heresy, but it has no training value.
If you are anticipating recoil, the best way to stop is to dry-fire.
If it is trigger manipulation, the best way to improve is dry-fire.
There are a couple of other techniques, but the explanations are rather lengthy.
Start with this:
Also try this: