Mind that there’s no direct way to change from stripe/mirror to raidz. It’s all:
- Replicate (implying having two full pools); or
- Backup, destroy and restore.
You could, however, add two drives to turn your two single-drives vdevs into two 2-way mirrors
This is what makes the boot drive disposable, and allows to reinstall without losing data…
It is possible, but fully unsupported, to hack your way to install on a partition, and then use another partition on the same drive for another purpose. Replacing drives then requires using the same trickery…