Help validating pool update (adding vdev and removing old vdev)

I’m in the process of specing out an update for my truenas core pool
Currently i’ve got a pool with a single vdev of raidz2 (10x2TB drives)
I would like to expand and replace my drives at the same time with increased density without the need to one by one update the drive, my current thinking is as follows.

Two new 18TB drives in a mirrored vdev under the same pool.
Migrate all data to the new vdev and remove the original raidz2 vdev (unsure about how this works in practise).

once removed add back some drives (maybe 6) in 3 mirrored vdevs

This should be possible from what i’ve read but want to make sure I haven’t missed anything.

No, it is not. Vdev removal is only possible if the pool consists only of single-disk or mirrored vdevs.

Ah so because it’s raidz2 it can’t be removed, is there any other way to get to a state where the whole pool is mirrored vdevs?

Would creating a new pool (with 2x18tb vdev) and migrating the existing pool work?

Yes it would. But going from raidz2 to 2-way mirrors is a serious downgrade in resiliency. Consider 3-way mirrors (at a cost, obviously) or make sure you have a good backup plan.

Since you seem to have the ports available, you can just replace all the disks in one go. Just start them all one after the other and it’ll be a single resilver operation. Same effect you wanted, with the advantage of being possible.

Yeh i’m ok with the resiliency downgrade, it’s mostly all data that can be re-downloaded (media server) and all my critical data has multiple backups (on-site and cloud).
For where I live the cost per TB is just too high to justify 3-way or raidz2 again, I got lucky with some second-hand drives before but this time I have to buy new.