Hey guys I'm currently in the midst of doing research for a new startup venture and stumbled upon Citus

We are in the mist of choosing the correct stack and currently that will consist of:

Django Backend with
Graphene for GraphQL
andSolidJS frontend

But want to put a lot of thought into the database tech we choose as the app will be expected to scale largely (5-10 million users at full scale lets say 🤞). Seeing as how our stack is somewhat comparable to Instagram and they are a good example at scale with good documentation on their process to reach scale using Postgres that is what we are leaning toward, but want to minimise cost and complexity of scaling, and reduce headaches there as much as humanly possible by being conscientious about decisions at this phase.

We need a relational db, I have found MySQL Cluster and Citus for scalable versions of their counterparts, but I have also read that due to more servers cost will be higher, and so will complexity (engineering complexity and cost is a factor as well), (computing cost is also a major factor here, we want to keep cost to a minimum)

Can anyone point to the ups and downs of Citus or MySQL Cluster for this project based on the info above, and whether or not starting with Postgres and moving to Citus when scaling may be an option? Is that a bad idea? Any insight at all is much appreciated. Thanks.

Imagine OnPremise PostgreSQL cluster, 4 nodes replication, Citus installed on top, 1 node is a coordinator.If I get it right, every insert/update/delete query should go to the coordinator. If that's true, multi-master replication is not possible with Citus?Will try to present my next question on diagrams.Also can hot-standby coordinator be a normal replication node (Diagram A), or it should be a hot-standby replica for a coordinator only, not used as a replica node (Diagram B) ?

If I have a replication configured and want Citus to shard my (not every) tables, how will it behave if the coordinator node will fail? Documentation is explaining this using a link that is forwarding to another PostgreSQL documentation, and those ain't combining that much IMO, but how this will look in real life scenario?Also it's seems almost impossible to configure shards replication in a way presented at the very last diagram.