+1 from me for network-manager-observe as this is core functionality for using network connected 3d printers.
Regarding removable-media, the guidelines for auto-connect of this take into account media editors / viewers which would include cura-slicer IMO, however since this is not published by the direct upstream project and is an unofficial build, I am not inclined to vote in favor of removable-media auto-connect in this case. So -1 from me for removable-media and hence mount-observe.
Note there are other options in this case though if auto-connect of removable-media is strongly desired, please refer to the guidelines I linked to above for more details.