I was eventually able to make GB6 (aka GB'11) run on Mojave by downloading the "GarageBand 11 Compatibility Sounds and Instruments" via Logic Pro X's Sound Library Manager. But I guess it would have also worked if I had replaced the GB's/Logic's Sound Library folders with symbolic links to one of the other partitions with a full installation already intact. (Because that's the whole "trick" how Logic's "Relocate Sound Library" command works. ;)
Nonetheless, GB6 then choked on Apple's preinstalled AU plugins on Mojave! D'oh! Its built-in AU Validator just hangs. I had to force quit GB6 again and again, until GB6 marked all plugins as incompatible.
Installing the lessons also fails. Obviously the custom built-in installer doesn't get any permission to run:
Installation of /var/folders/jt/ctnf15_170sb8btbc5ysfskc0000gn/C/com.apple.GarageBand/downloads.apple.com/static/gb/gb11bc/GarageBandBasicContent.pkg didFailWithError:Error Domain=NSOSStatusErrorDomain Code=-2147409654 "CSSMERR_TP_CERT_EXPIRED" UserInfo={SecTrustResult=5, NSLocalizedFailureReason=CSSMERR_TP_CERT_EXPIRED}
Which means: I should have looked at the Console before, ha!
https://downloads.apple.com/static/gb/gb11bc/GarageBandBasicContent.pkg
That's what's missing.
You can then run the standard Mojave installer to install it separately.
GB6 will then still report an AU validation error on launch and ask for the lessons download. Just click Cancel.
To remove the Lessons download prompt for good, you may need to open ~/Library/Preferences/com.apple.garageband.plist with a plist editor (use e.g. Prefs Editor or the Terminal if you know what you're doing) and remove the "downloadStarted", "numRetry", "pkgName" and "url" keys from the "DfPrefs_ActiveMentorDownloads" dictionary. (For those folks who don't understand what I'm talking about: better just leave it alone for now. :)
Other than that, GarageBand 6.0.5 works on Mojave.
Have fun!