MacPro, APFS, Bootcamp Startup Disk
Trying to get a "more recent Mac's bootcamp installer" is the wrong approach to take in a effort to solve your problem. Especially, since there are other ways to solve your problem which involve using the rEFInd Boot Manager. This free software is capable of instructing the firmware to boot Mojave.  

One solution involves creating a small 300 MB "Mac OS Extended (Journaled)" volume where rEFInd is installed. Here, rEFInd is configured to appear as a legacy OS X operating system. The Boot Camp software installed in your Windows operation system will detected this volume as OS X. When booting from this volume, rEFInd will automatically instruct the firmware to reboot to Mojave. This is setup to happen silently. 


  Note: The rEFInd Startup Manager executes silently and therefore does not require user input. 


An example of how to setup rEFInd as described is given as the accepted answer to the question: macOS partition disappered after installing windows 10 using bootcamp.