A small side-note to solve your "unique [EFI-SP] directory to that OS instance":
In each GRUBified OS instance, in /etc/default/grub (or on Debian and derivatives, to avoid altering the distro-shipped config file, /etc/default/grub.d/local.cfg ), set:
GRUB_DISTRIBUTOR=
This is used by grub-install.
If calling grub-install directly one can also pass --bootloader=id=
In each GRUBified OS instance, in /etc/default/grub (or on Debian and derivatives, to avoid altering the distro-shipped config file, /etc/default/grub.d/local.cfg ), set:
GRUB_DISTRIBUTOR=
This is used by grub-install.
If calling grub-install directly one can also pass --bootloader=id=
The value is set via efibootmgr's --label