thanks for your elaborate answers.

I think I have figured it out. Main problem was my misunderstanding where to put the dem-options. If I put them correctly into the batch file which I use to generate the elevation .img files I get a nice hillshading in Garmin Basecamp. Still missing is the possibility to generate elevation profiles in Basecamp (using tracks generated in Basecamp) and to see the "height" on any position using the mousepointer. For this I have to puzzle harder or does somebody have any idea?

Here I include my working command line as it might be interesting for the community. This results in a collection of .img files containing elevation contour lines and hill shading:

java -Xmx2000M -jar [...path to mkgmap ...]\mkgmap.jar --show-profiles=1 --dem=[...path to hgt file folder ...] --dem-interpolation=auto --dem-dists=3312,3312,13248,26512,53024 --style-file=[...path to mkgmap style...] --transparent --draw-priority=20 --family-id=30 --mapname=90300001 [...path to collection of splitted elevation data files ...]\9030*.osm.pbf


Regards,
Bernard