There's various options... my current direction is CockroachLabs for the DB side... but Cloudflare offers D1 (sqlite interface over durable workers/kv) which is

There's various options... my current direction is CockroachLabs for the DB side... but Cloudflare offers D1 (sqlite interface over durable workers/kv) which is kind of what you are talking about in practice. Actual data sharding, without persistence in different regions is more difficult. Different database systems lend themselves differently for different loads and sharding characteristics... I've seen MS-SQL in master-master across the globe, postgresql with sharding that is replicated to base host, and various ScyllaDB/Cassandra setups. It's always a mixed bag.