Well, recommended…
Likely contributed by someone, but it seems opinionated and I don’t necessarily agree.
grub was updated to 2.12rc1 for 23.11, which has caused some fallout: grub2: 2.06 -> unstable-2023-07-03 by K900 · Pull Request #240887 · NixOS/nixpkgs · GitHub
Your setup uses a zfs /boot pool, which provides little benefit yet requires you to use GRUB’s broken zfs support for it to be able to find your kernel/initramfs.
I would suggest following the recommendations outlined in the NixOS manual, that is mounting your (fat32) ESP at /boot and using systemd-boot instead of grub, i.e. set boot.loader.systemd-boot.enable = true
EDIT: maybe-related Boot broken on Raspberry Pi CM4 when using linux_rpi4 kernel package · Issue #270343 · NixOS/nixpkgs · GitHub