After waiting with my 10.12.6 iMac 27" 2011 with a RAID 0 boot drive for 2 YEARS, I've finally given up (since 10.14 doesn't support this Mac). I purchased a 1TB SSD and an 8TB HD and am installing 10.13.6 on it now. Ugh.
I think I have a workable way, over on the Mojave forums. I certainly didn't have any issues installing HS 10.13.6 or updating it to Mojave DP1 - I've outlined the process there.