I manage a large sized Postgres farm with 100s of instances, and there has been one case where we need multi-master, and I went with Galera cluster for MariaDB. You can shard using the citus extension for Postgres.
Depending on requirements, there are an increasing number of options for "active-active" Postgres deployments. A colleague wrote this on a federated active-active configuration on Kubernetes: https://info.crunchydata.com/blog/active-active-postgres-fed...