You can just expand BaseSystem.dmg then replace "Install macOS High.Sierra.app" on the root with the full version (>5 GB).
My CHSI.tool automates the process for creating an High Sierra ISO - see post#30,
CSI.tool for Sierra - see post#13.
If you want to prevent APFS conversion on virtual SSD, you can use "startosinstall" from terminal to set --converttoapfs NO, having booted into the High Sierra ISO installer - post#32.
Many thanks! I will try it.
I am fixing my old creating ISO script supported 10.6 to 10.12 versions.