I had a double post of my question in the 'Instructions for patching ...'-Thread - I thought I deleted it ... but I got help there ...
jcn363 posted a solution to reduce the bottom margin here:
https://www.mobileread.com/forums/sh...03&postcount=5
I deleted the fullscreen-string in eReader.config and put the suggested code by jcn363/GeoffR into nickel.yaml. Note: you have to enable the bottom margin to get the smaller margin without the text ...
I'm very happy with the result. The margin is much smaller, nearly as before. I can live with that.
Thanks for your help!