Greetings Stefan, I’ve seen some of your posts on the Ardour forum, good to see someone being so active / pro-active with Redux and Ardour on Linux.
I don’t think it’s possible to set the default library and it’s shared with Renoise, however I think you can set the shortcuts on the right of the screen as a workaround. I’ll have a look on my laptop when I’m on lunch break but in the meantime see if these are helpful;