You need to buy the same exact drive with the same capacity and speed. Your raidz vdev be as small and as slow as your smallest and slowest drive.
btrfs and the new bcachefs can do RAID with mixed drives, but I can’t trust either of them with my data yet.
Tbh the idea of keeping backups defeats the purpose of using RAIDZ (especially RAIDZ3). I don’t want to buy an LTO drive, so if I backup, it’s either buying more HDDs or S3 Glacier ($$$). I like RAIDZ so I don’t have to buy so many drives. I guess it protects you if your house burns down, but how many people do offsite backups for their personal files? And dormant, unpowered HDDs die a lot faster than live, powered HDDs.
It doesn't have to be the same exact drive. Mixing drives from different manufacturers (with the same capacity) is often used to prevent correlated failure. ZFS is not using the whole disk, so different disks can be mixed, because the disk often have varying capacity.