I know this is a recurring theme here on MR but here goes: I have 10GB of Other on my Space Grey 6, what could it be? It is jailbroken, and is there any Mac app that will show me what in part of the Other partition?
This is probably apps' cache. Try the Battery Doctor app: it has a clean cache option. If it does not work, you need to recover.
Do you have a lot of visual voicemails? At some pint that accounted for many GB of "other" on my iPhone.