Thanks for your perspective.

I decided to go with the new vdev because:

• That way, I have access to all 5TB (1TB from the old SSD and 4TB from the new one).
• One pool is easier to manage than two.
• I know that I don’t have auto-healing, and if either of the SSDs fails, I lose the whole pool, and I accept that.

I did this via:
zpool add ssdPool /dev/disk/by-id/nvme-Samsung_SSD_990_PRO_4TB-part3

I don’t like ext4 because it is not easily backupable (it doesn’t have snapshots, which are moved periodically to hddPool).