It seems can't specify the target volume, but not sure if --applicationpath can assign the target path.
Anyway, what if
1) only leave a HFS+ formatted SSD installed (pull out ALL other hard drives)
2) boot from USB installer
3) run that command line (with the correct "Contents") but do not specify a target volume?
I will try that method later just don't know why it's so hard to do when I did exactly like in this guide for example:
https://blog.macsales.com/42884-is-it-possible-not-to-convert-to-apfs-when-upgrading-to-high-sierra
- Will be back later with results...