Is it possible to build lightweight version ISO of Qubes 4.1?

  • dom0
  • sys-net - debian-10-minimal
  • sys-firewall - debian-10-minimal

make iso about ~500MB and use it on old hardware?

Yes, you can customise the build scripts as you want.

You would want to create a minimal template loaded with appropriate
drivers and Qubes networking packages, so no longer so minimal.

Alternatively, during the normal installation, you can choose to not install the Debian 10 template (that option seems to be in software selection).

After you login to the new installation of Qubes OS, install, update, clone, and customize the fedora-33-minimal template. Replace the fedora-32 template by the minimal template. If something goes wrong, you can switch back. Finally, delete the feodra-32 regular template.

If you choose to use the Qubes-builder, you have to at least work around this:

and maybe this:

You don’t need to work round these issues, because the builder gives you
the chance to use packages built by Qubes rather than building your own.
You would slipstream the Qubes packages in to your build, and customise
the iso configuration.