How funny is that: cross-compiled the first time. Kernel for my #Librem5 is working. I'll have to see if a merge request to include btusb by default is accepted in its repo...
Hm, o.k. connection to the headset broke after a few minutes and was hard to recover.
Anyway, it looks like the software stack for #bluetooth of #PureOS on the #Librem5 is not the main problem and the built-in #redpine adapter causes most of the problems.