Do you have a known, good backup of all the data in the pool or somewhere to store it temporarily? Also need to know if NAS is storing data only or if you have apps and VMs, etc
Destroy pool and recreate with new, smaller drives is option. Reload all your data.
If you can return new SSD and get larger models and replace one by one, is another.
Depending on where you are in the world, Best Buy in USA has a WD external on sale