If you think 20TB "isn't that big" I want to know what size of DBs you're working with 0_0
It's big but it's not so big it wouldn't fit on SSD on one particularly beefy server (two for redundancy). Sharding this would be more about the transaction rate. Actually, sharding would always be about the transaction rate.
It doesn’t even remotely need to fit on one SSD with logical volume management (or RAID).
I mean yes, for a single DB it's large, but if you're thinking about sharding you're probably in the tens of TBs, and if you're a company offering sharding you've prob sharded larger workloads.
It's really not that big for a postgres db in a lot of places, honestly.