Smooth update, eventually they’ve fixed 90° displays bug.
1. Updated Clover 4910 → 5070, it broke everything (pretty often situation, not surprised). Found out there are breaking changes, so I moved all my *.efi files from drivers64UEFI directory to the new DRIVERS/UEFI directory. (See attached images below.) It didn't help, then I deleted the original directories (drivers64 and drivers64UEFI) and it worked.
2. Updated macOS 10.14.4 → 10.14.6 through Settings, it went smoothly. I left the computer for an hour, and when I came back it was all done. I have no idea how long it took and how many reboot attempts it did.
That "new" 2020-003 supplemental security update for 10.13.6, dated June 2, will change its build number from 17G13033 (from the May 28 update) to 17G13035. It's different from the earlier one, obviously, and I don't know why Apple didn't rename that a 2020-004. (On my "Mini ITX 3" and "Mini ITX 1" computers (below) both running 10.13.6, the latest one runs fine.) If there is a "new" Mojave update, I haven't seen it.