Just a heads up for anyone reading this, as a mindfulness coach I wouldn’t recommend this technique. The rubber band and “stop” voice are pretty harsh and rather we want to practice inner kindness and letting go.
For some the "rubber band" method may just be a silly reminder, but for others it could be a mild form of self harm. The same person may be in a very different emotional space from hour to hour or month to month.
"Counting five breaths" is a tool I got from some mindfulness resource. Focus on the breath as much as you can and count the exhales. When you get to five, begin again from one. Repeat until settled. If I'm fairly out-of-it, it may take me 10 repeats of counting five breaths to center myself. Sometimes it doesn't work at all.
I will say that I have been practicing mindfulness meditation for 7 years in various traditions and have never come across this technique. It also goes against the definition of mindfulness (paying attention to the present moment without judgement) as it is applying judgement to your thoughts.
I have experienced very difficult and dark periods of meditation when I was hard on myself for thinking, and learning to be very kind to myself in meditation was a huge step forward in my practice. Rather than “stop”, I say “thinking” to myself in a kind way.
None of the techniques I described are meant to be judgemental. It's only about helping yourself passively recognize rumination. While it did them I never once felt I was being unkind to myself, since it's neither meant to hurt nor negatively judge. The "STOP" example is literally about picturing something, not about "screaming at yourself".
To get back to your initial points - you are definitely correct that you have to approach yourself with kindness, and if you feel that anything I described is unkind, you should find a way that works for you!