Ithaca Could you elaborate on tension leading to plonkiness? After 20 years mostly playing on a digital, my touch has gone to h***. (When I'm trying to play something light and delicate, it still sounds like an elephant attempting ballet. It's appalling.) I was pretty sure it was because of all the shortcomings of the digital, but now your comment has me wondering if it's also my uncontrolled tension. Tension -> plonkiness would explain why the only time my touch seems to improve at all is when I'm playing super slow and super quiet - under those circumstances I can make sure that every note is played carefully with proper alignment and no excess tension
I don't play with a perfect technique by any means, but I think I'm not too bad because I never face tiredness/pain. So hopefully, my advice would be helpful. 🙂
I think of the minimum possible effort needed to do something. For that, you first start with approximately the least effort you can use to play something at a given moment. While you're playing it, think starting from the fingertip. If you are raising the finger and striking down, that is "unnecessary effort". Then, go over to the wrist. If you are straining the wrist to keep it in position as opposed to keeping it relatively loose, that is unnecessary effort. Then, the forearm. If you are tensing the forearm, it could probably be solved by aligning your hand better (so that it's not at an angle, or too high or low). Then, the upper arm: if you feel like you're using your biceps or triceps to "hold" the notes, that is generally unnecessary. Then, the shoulder and upper back: if you are raising your shoulder, that is unnecessary effort.
First, try to do the best you can right now. That is your baseline. Now, there is only so much that you can control voluntarily at a given moment. Sometimes, for instance, I will think: "Keep the pinky low! Why are you curling it?!" But it keeps curling. That is because the brain associates whole movements to each other, and doesn't have full independent control of each muscle in the body (and never will). This is called "co-activation". This is what we end up fighting when trying to avoid tension, because the set of movements involved in playing the piano are "not natural" for the body. For instance, a common thing you see is that if someone has been knitting for several decades, their fingers likely tend to fall into that sort of position out of habit. These habits will not resolve themselves instantly, but the first step is to be aware of them.
Once you are aware of them, there is a gradual process which takes several weeks which allows you to rework those movements. You would try to reinforce what you think is better (in my case, to not curl the pinky) in a number of different ways. The goal is not to be perfect; it is to constantly reinforce the movement and do it a little better every day. How much better it gets each day is variable; some things can click very quickly, while others can take months or even years. But it certainly does get better.
Now, forget what I said above; all of that is just the mechanism. What would you do in practice?
Your playing is controlled while at a slow tempo. I would suggest playing small chunks, like one measure or phrase at a time at a moderate tempo while trying to keep the same relaxed sensation in your hands. (I wouldn't go up to a fast tempo because the movements change, but at a moderate tempo, they will still be similar enough to a slow tempo.)
Once you are more familiar with how it feels to play with less tension, another suggestion would be to imagine how you want it to sound. You want it to sound light and relaxed, maybe like a Mozart piece on the piano. It is a very useful strategy to just focus on the sound and try to "let it happen". (Molly Gebrian mentions this too.)