You could use 4x 2-way mirrors with a single spare drive as your final topology. That would allow you to add two drives each time you need more space.
Not using ZFS is not an option. Mirrors are ZFS, too.
A solution has been accepted into the openzfs repo but not yet available in any build as far as I know: raidz: fix space accounting after expansion by Skountz · Pull Request #18324 · openzfs/zfs · GitHub
If I interpret the review of this fix correctly, the fix does something during the expansion as well, so it won’t fix the reporting of expansions that happened, before the fix was present.
It actually works on previously expanded: RAIDZ2 Expansion: Unexplained Space Loss & 2x Overhead · Issue #17784 · openzfs/zfs · GitHub
So just to confirm my understanding after reading the threads and doing the tests myself.
truenas_admin@truenas[~]$ zpool list P01 && zfs list P01
NAME SIZE ALLOC FREE CKPOINT EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT
P01 2.25T 2.18T 70.4G - - 1% 96% 1.00x ONLINE /mnt
NAME USED AVAIL REFER MOUNTPOINT
P01 1.06T 0B 151K /mnt/P01
The issue only lies in the estimation/calculation/reporting, but not actual capacity on disk, correct? If I keep my original plan of expanding the VDEV with more disks over time vs. buying them all now, I will not lose space in both cases and will have the same capacity, but will only have incorrect size of files and disk availability in the case of expansion?