Hey thanks for the response. I think I found a solution to that problem you just mentioned.
So - following the normal custom installation page I linked above, just simply modify the creation of all the LVM volumes to the following:
cryptsetup open /dev/sda2 luks
pvcreate /dev/mapper/luks
vgcreate qubes_dom0 /dev/mapper/luks
lvcreate -n swap -L 4G qubes_dom0
lvcreate -T -L 20G qubes_dom0/root-pool
lvcreate -T -l +100%FREE qubes_dom0/vm-pool
lvcreate -V1G -T qubes_dom0/root-pool -n root
lvextend -L 20G /dev/qubes_dom0/root
mkfs.ext4 /dev/qubes_dom0/vm-pool
The final line in that formats vm-pool to ext4.
Upon reboot and then reloading the installer, when you select custom layout and set up all the mounts vm-pool exists and when you select it, it DOES NOT show as formatted as ext4, however, if you just leave it and start installation, the full install process seems to work properly and when you get to the VM installation part (installing fedora-33 template) it does seem to install it to the correct location (vm-pool) and no complains about not being ext4.
And finally after rebooting into fully installed Qubes - checking the “disk space monitor” icon in system tray is now properly showing I have vm-pool with templates appearing to be in there, and my root system is using 4.3GB - this all seems proper.
When I run the “lvs” command as root, the layout I get looks identical to the one I screenshotted above from an “automatic” installation.