Those are two completely non overlapping use cases. If you can use pgsql for your problem, you have no business trying to use a distributed key value store instead. That would be at least as dumb as driving screws with a hammer.
Of those the first two are not PG, they just share the wire protocol and try to be compatible at a SQL level. Aurora is not really distributed, it's replicated for availability and durability six ways at the block storage level. Citus is distributed as I understand it though.
> but it's also reasonable to come at it from the other direction - build a SQL engine on top of a solid distributed key-value store.
Sure, that's possible. I'm not talking about the wisdom of building your own relational database as the end goal, just that a distributed key value database and a SQL database don't have overlapping problem sets.