does one always have to increase

As you can same by my numbers, me too. ;)

The name of the exercise isn’t important. What’s important is how much pressure/stress you put, for what amount of time.
I can do clamping, but the pressure I put may vary in a scale of 1 to 10. And for different durations. I can perform a much forceful jelq than a light clamp.

So briefly: yes. You will have to increase the intensity and time in your exercises in time- once you get adapted to a level. Because that level of intensity won’t give you further gains.(after a while)

I wish you could just use visualisation techniques to get gains

Mental success is nice, but it isn’t enaugh :)

I think visualization helps a lot.

In an actual physical sence or in the sence that it helps one maintain the willpower to continue to work to a goal?

Both.

One more time jelqbeginner, use the spell check.