Thanks for advice, Skywatcher. I made the mistake placing them most of the time indoors at a south window with bright sunlight in an enclosed container very humid and warm. I think I've overdone that a bit with all of my seedlings.
I learned that they are not tomato plants and hungry for light - they are shadow loving plants. So after 3 months I kept all of them in the shadow zone on the outside the whole day.
I potted in a mix of clay, lava, purmice granulate with a little bit of peat. The peat is for the roots, they would not find a hold on the fine mineral granulate itself without a bit of soil.
I'm concerned that the plants will not recover from the sunburn after all of the small mistakes I have made, we shall see.
Also the sun has changed somewhat. Even my year-old mammilaria get sunburned when exposed to mid-day sun at places on the west side of the house where the plants would normally enjoy the long afternoon and evening sun, nowadays it's simply too much for them. So far I had no losses because I am always on watch what the best places would be. My Lophophora diffusa required a place where it has 2 hours of the late afternoon sun, not more. Other lophs I had to place on the east side indoors also only 2 hours sunlight but not at noon.
I think I will do the same with the trichos when I don't see any progress outdoors, I will simply chose the places I found for my lophs. The younger plants are quite sensible to light I didn't take that into consideration from the beginning.