I am just trying out the Free Trial of SD.

Here in southern Germany I am unable to see the standard Airspace C starting at FL100.
I can see C starting at FL130 in the alpine area, but not the standard C in the flatlands.

Same goes with airspace E. Visible when the lower limit is off-standard (like 1000ft)
but not when standard (2500 ft).

What am I doing wrong? Thanks a lot!

(and yes, I have well planned a route high enough - currently FL150)

Yes, but authorities don't always allow transits of such airspace in every country, so I had to ask.

The main reason we omit such large country-wide airspaces at the moment is that they slow down the system. Not when drawing on the map (they are typically excluded from the map anyway) but as you fly, we need to check every second whether (and how) your flight intersects with these airspaces which typically comprise thousands of coordinates.