Using a 2.5” drive in a 3.5” slot works, but feels a bit wasteful. Being able to upgrade the existing backplane to include proper 2.5” drive slot would be fun. Either with a mix of 2.5” and 3.5” slots, like in the Hybrid Storinator, or a full bank of flash like the Stornado v1.
Adding more slots would require an additional HBA/cable/power/etc. Assuming the current backplane can be removed it should be theoretically possible to swap in different configurations.