I'm not aware of any requirement to double drives when upgrading ZFS storage.

What kind of upgrade are you talking about?

If you're thinking about adding another vdev to a pool, you're free to set it up in whichever layout you prefer, mirror, raid, etc. This way your zpool would consist of your old vdev + new vdev.

If you wanna upgrade your existing vdev in-place, you could offline one drive, replace it with one with larger capacity, let ZFS resilver, then repeat it for all your drives. This way your zpool would still consist of your original vdev, but with larger drives. I used this to upgrade from 4x2TB to 4x4TB (I have room for max 6 drives).

If you're patient and can wait for this PR to be merged (it is open since June 2021) and integrated into TrueNAS, then you would be able to just add drives to your existing vdev. I'm waiting for this to add 2 more drives to my 4 drive vdev.

Cache and log drives can be shared among the vdevs in a zpool.

*cache...