Thanks to both of you for your replies. I leveld the printbed several times, because it has 3 screws and I found out, that I have to repeat the leveling at least 2 times. Not really a big problem.

At the moment I have the printbed as it arrived here with a blue print tak on it. Not the tap, but a sheet of plastik that is exactly as big as the print bed. This works pretty well, when I do smaler area prints, but it fails on a biger flat print. The left side always warps up. I print ABS (that one, that came with the printer) at 225C and the printbed at 105C. Layer height 0.18 first layer 0.27, Bas print speed 60mm/s.

I will try to use the glass plate, that came with the printer as an extra gift, but have to print the holders first. I guess it is good to just have the holders at each corner preventing the glass from sliding around and there is no need for holding it down? I got 2 paper clips with the glass, but I cannot use that, because I do not find a place to clip it on, where it is not an obstacle for the printing head.

I already removed RG. Will stick with Flashprint for the moment and made the update to 3.13 today.

I do not really want to print directly via USB, but I would like to be able to copy the files to the SD card in the printer via USB. That would be nice, as I would not have to remove switch the card from the printer to the PC and back.
So if there is a way to copy the files via USB to the printer, please tell me how to do it.

For maintenance, I'll do some lubrication with silicon oil or something like that and keep an eye at the screws.

All in all, I am very happy with my new toy, but still a lot to learn.