Best practices for preparing an application to (possibly) be sharded (FDW) in the future?

Just out of curiosity, is there anything that we (as developers) can do on our code to (all together):

- Have maximum performance while running on a single PG 14.1 database (HA) (Google Cloud SQL);
- Be prepared (if needed in the future) to migrate the database to a sharding solution (FDW) once the microservice exceeds the capability of a single machine
- Don't make the code (that access PG database) to be much more complicated compared to one running on a SINGLE Database (ho sharding)

- No need to change the application's code when (and if) the database needs to be sharded in the future (FDW built-in approach).

Our use case is to use PG today and be prepared to scale beyond a single Google Cloud SQL machine max capability (maybe self-hosted Postgres when sharding is necessary)
We are a financial institution/Fintech with over 1MM customers (and growing 50% YoY). The new platform will probably be built on top of PG (as the standard database).
One PG (Cloud SQL) for every microservice.

Any help or suggestion would be appreciated.

Thanks