It looks like you have a 2TB NVMe, and your ZFS install is similar to the partition install (with a boot pool, bpool, and a root pool, rpool, on paritions) shown for Arch on the OpenZFS site – and you have a 500MB EFI partition, that (if it’s also like the OpenZFS Arch install) likely has an EFi grub install that boots from UEFI, and then loads the bpool, which mounts the rpool.
You’ve got lots of room on your disk – you’re only using 12G of your 1.8T root pool? – but, the bad news is that you can’t resize/shrink that rpool that to make room for a Windows partition. You’re going to have to back it up/recreate it/start from scratch/etc.
Before getting to that point, I’d recommend first learning/setting up rEFInd:
[The rEFInd Boot Manager]
on your system, so you can use it to boot into your existing EFI grub.
Then, once you’re comfortable with that process, try adding:
ZFS Boot Menu
[GitHub - zbm-dev/zfsbootmenu: ZFS Bootloader for root-on-ZFS systems with support for snapshots and native full disk encryption]
to your rEFInd, so you can boot from rEFInd, to ZFS Boot Menu, directly into your ZFS Linux (bypassing the EFI grub) – in addition to booting from the EFI grub).
Once you’re an expert at controlling your boot environment, then you can think about how you want to redo your drive and install your desired OSs, like maybe you can make a;
EFI – 900MB
Windows partition – .8TB
Linux ZFS – .8TB
or
EFI – 900MB
Windows partition – 500G
Linux ZFS – 500G
Data partition - .8TB
etc.
and setup rEFInd, ZFS Boot Menu, etc. to choose between whatever you setup.
Sure, occasionally, an update or install might hijack boot control away from rEFInd, but once you understand what’s going on under the hood, it’s usually just a simple fix to restore the boot chain as desired, e.g.:
$ sudo efibootmgr -o 0000,0085,0002,0003
BootCurrent: 0000
Timeout: 0 seconds
BootOrder: 0000,0085,0002,0003
Boot0000* rEFInd Boot Manager
Boot0002* Windows Boot Manager
Boot0003* Windows Boot Manager
Boot0085* ubuntu
– The rEFInd Boot Manager: Keeping rEFInd Booting