Terminal command to force-install Mac OS High Sierra onto a specified drive?
This is independent of the RAID problem and may be independent of any APFS problem as well.

The core of this question is that the commanline will not allow you to redirect the install away from the booted drive. Therefore this answer covers the question as presented in the question headline.

The startosinstall command offers the switch --volume to redirect the installer onto a specified volume.

Try it:

..path/to/startosinstall --usage


and find that option missing.

It is there for any High Sierra installer. 

BUT

This option is only presented when SIP is disabled!

The -–volume argument will be hidden, because startosinstall requires – at least – either CSR_ALLOW_UNRESTRICTED_NVRAM/0x40/64 or CSR_ALLOW_ANY_RECOVERY_OS/0x100/256.
Without one of these, the --volume argument won’t be supported.

That means also that using that switch from the Recovery partition it will be present every time.

This trickery is documented at Comparison of startosinstall's available options depending on boot OS environment

So in the case of this question:

/Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/startosinstall --converttoapfs NO --volume /Volumes/YourChoice