How do I automate network installation of many Ubuntu 18.04 systems with EFI and PXE?

If you really want to do individual config file based on mac address, you can make the contents of your grub.cfg something like

if [ -s ${prefix}/${net_default_mac} ]; then
    source ${prefix}/${net_default_mac}
fi

Personally, I find grub much more flexible than pxelinux because the grub config can use variables and logic branches. I tend to keep a generic grub.cfg that uses variables that can be set based on the machine attributes.

Here is a small example

set my_interface="auto"
if [ "x${net_default_mac}" = "x01:23:45:67:89:ab" ]; then
    echo "using variables for template"
    set my_label="template"
    set my_preseed=tftp://${pxe_default_server}/preseed/preseed.cfg
fi

if [ -n "${my_preseed}" ]; then
menuentry "Xenial Install - automated - ${my_label}" --id=install_xenial_unattended {
    set gfxpayload=keep
    echo "ANY EXISTING INSTALL WILL NOW BE OVERWRITTEN"
    echo "THERE WILL BE NO PROMPTS"
    echo "LAST CHANCE TO ABORT (poweroff to abort, wait or hit esc to continue)"
    sleep 10 --verbose --interruptible
    echo "Loading Kernel..."
    linux /ubuntu/xenial/amd64/ubuntu-installer/amd64/linux netcfg/choose_interface=${my_interface} auto=true priority=critical url=${my_preseed} ${my_kernel_install_args} DEBCONF_DEBUG=5 ---
    echo "Loading Ram Disk..."
    initrd /ubuntu/xenial/amd64/ubuntu-installer/amd64/initrd.gz
}
fi

You can also combine these techniques so that you can put variable assignments into a file named for the mac address so they get loaded dynamically.