o.k. "Boolean Difference" is the magic!
Did a good progress with the tips you all gave me!

The main mistake I was doing all the time, was, that I tried
to get these shapes with curves/profiles and the "trim" command.

When I use "boolean difference" instead (thought this only works with "solid" objects)
I can make fillets without problems, because doing this was I end up with proper closed shapes.
What also helped much, was the tip to make the profiles harder first and get the rouded corners
with fillet later!

thanks so far!
Mike