The logger is now redone in KiCad:
Hardware bugs should be fixed, the bluetooth module can now be soldered directly to the backside of the PCB, there are mounting holes for the enclosure (if the battery allows it) and the whole board shrank in the process. Will be on the way to my mailbox soon.
3.3V rail distribution is still not too great, but for any major improvement of that, the board should have been 4 layer, which costst a whole lot more.
I did consider other more current efficient modules than the HC-06, but for now I know it works and is readily available at every Chinese silicon corner. It would have been fun to get acquainted with Nordic's nRF5xxx SoCs, though.