Some things to think about.
Even ZFS now let's you add a new drive to an array, and the sweet spot for $/TB is ~16-20TB at the moment, so maybe think about 2 or 3x16TB and add more later (also less power).
Consider manufacturer recertified (not refurbished) server drives from serverpartdeals.com or your local equivalent, after all RAID is there to let you survive a disk failure, it's treated me well, and lets you avoid SMR drives.
You can mix drives of different sizes if you use Unraid or roll your own with mergerfs+snapraid (+OpenMediaVault perhaps). I do the latter, it's a bit of a setup, but has the advantage that drives are just drives and you can use the working ones while rebuilding the array and you can recover accidental deletions (for a while), which brings me to 'RAID is not a backup'.
For true data safety you should have an offline backup (i.e. drives that live disconnected from your computer except during backup, safe from lightning, accidental deletion etc.) and eventually an offsite copy.
Personally I think the AI bought all our drives from WD is likely BS (seems lightly supported) to goose their product prices, so hopefully it'll blow over, but prices seldom go down, inflation catches up. Sigh.