Ok was able to install onto apfs external hd usb 2.0 from a hfs partition with clover 4091 and core .pkg method..
All works except brightness control(same as in Sierra) and an empty trackpad pane.
While installing, the installer near its end when it says optimising software always failed saying something went wrong. But I still booted and reached the desktop.
The boot time to reach the desktop with apfs and usb hdd for me was 11.5 minutes.