Sharding databases is not such a dark, magic art. Citus relies on Postgres for many key features; and Citus does the rest. It's already quite "feature complete".

If Citus would become proprietary overnight, my main concerns of maintaining a fork would be around the codebase and the language expertise more than the sharding concepts.

Note that sharding is different from a purely distributed database. The latter is an entirely different class (and more complex system).