Thanks for your reply!
As you can see it took around 5 hours to do the backup of that disc which had 4:20 h of material on it - so I can't complain about slow encoding even with the "Highest (Very Slow)"-settings.
I just did a scan of the main movie's video bitrate with Bitrate Viewer - and it says the average bitrate is 4.058 kbps which exactly matches BD Rebuilder's calculated bitrate from the logfile. Maybe I'm trying it again with a custom target size of 24500 MB and see what happens.
By the way: Although the bitrate of that backup was quite low BD Rebuilder did a fantastic job when it comes to the picture quality which is still quite good! Wasn't expecting that when I saw the calculated bitrate but that tool keeps still surprising me in a very positive way.