Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Thinking about this a little more, the most reasonable solution seems to be to simply abandon Fusion all together. I should probably just split the fusion drive into Macintosh SSD APFS formatted and Macintosh HHD HFS+ formatted. If I do that I can have Mojave on the APFS SSD and update it anytime without any issues. Any large directories like Music and Pictures etc. would just go on the HDD.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.