A raid z2 setup should work well with 6 ssds. A weekly scrub will make sure no data is weak/ gets corrupted. Scrubs are only reads and should have basically no effect on the drives. The dual parity will make sure your data is safe.

Get a HBA flashed to IT mode and use that.That will expose 8 ssds to the system directly. You can then use a sas expander with the HBA to connect even more ssds. The expander only needs pcie power and a connection to the HBA. No need to upgrade unless you want to upgrade. Ssds don’t need to be mounted so you can easily just shove them anywhere in the case. Some airflow is recommended but not necessary.