Raidz expansion questions about parity

Hi,
I am about to switch from my regular ubuntu with some drives to truenas, once I upgrade to 2 drives.
After reading some info about raidz expansion. While it sounds cool the main drawback is that the already stored stats will stay with the old %parity data loss
e.g
4 drives raidz2 = 50% parity data
8 drives raidz2 = 25% parity data

even though with the about to be released 24.10 the already written data wont get the effect of less parity data usage.

So my question, lets say at 1 point I will upgrade from 4 drives raidz2 → 8 drives raidz2
Can I use a 9th drive (with external case and most likely ext4 as FS) kinda use as a drive to basically copy data which is stored with 50% parity
re write it to the new 8drive raidz2 pool and then remove the copied data (probably with some renamings involved)
The idea is to obviously get all my data written with parity 25% and fully utilize all my 8 drives

The lost overhead can be recovered simply by reading and rewriting the data. (With the one caveat that there are some reporting related issues that persist and are being investigated elsewhere on the forum–but the capacity is there). I’d suggest taking a look at: