The Elektron Digitakt and all of its features works magnificently with the Axo via USB and Din. The Novation Launchkey Mini MkIII also works without issue (including the Launchkey's built-in arpeggiator!).
Have been running midi via the UART pins and no resistors without issue. Mentioning that because it was sort of unclear whether they were required for UART in the same way that they are for using the pins behind the Din connector. Though, I'm not TX from the Axoloti so maybe things will change and will require the resistors and octocoupler when I do?