The Bunker Group
Your assumptions are not incorrect....except for one.The RAAF requires an airborne refuelling platform that incorporates a boom. At the present and for the foreseeable future, the only platforms with that capability are modified airliners. I very strongly suspect that the $17.5 to $26.2 billion price cited for the KC-30 replacement is the WOLC (Whole Of Life Costs) for the capability, not the flyaway costs. If it was just the flyaway costs that would be something like 30 - 40 aircraft and I can't see any justication whatsoever for a number of platforms that high.
There is no guarantee that the RAAF is only required to provide a boom refueller. Future capabilities may require drogue. And the FSP does state the AAR fleet will be a crewed/uncrewed mix. These allow for two types of AAR