Installing desktop version through auto-install d-i/pressed doesn't work

I’m not sure if documentation is accurate regarding autoinstall/preseed (apb)…

I tried, but failed.

Which documentation are you referring to?

You also mention “using cubic on ubuntu 20.04 update to generate the iso”. I have never heard of this tool before, presumably this is which is hosted in Launchpad but has never been submitted for inclusion as a package in Ubuntu and the Ubuntu team cannot vouch for it.