@ledOnion You can easily implement it.
Delete code in this file /etc/grub.d/40_custom.
Then open /etc/default/grub and add option rootovl in the end of line GRUB_CMDLINE_LINUX=
Also increase dom0_mem=max: size (for example, dom0_mem=max:10240M or dom0_mem=max:20480M)
Then update GRUB
sudo grub2-mkconfig -o /boot/grub2/grub.cfg
Now, the default Qubes boot will launch overlay mode. To start persistent mode, press E in the GRUB menu, remove rootovl and press F10 - it will launch persistent mode for a single session. If you need to run zram mode, replace rootovl → rootzram .