We shard geographically, with a single multi-tenant Postgres schema per geographical area. You run into corner cases when a customer wants to operate into two

We shard geographically, with a single multi-tenant Postgres schema per geographical area. You run into corner cases when a customer wants to operate into two distinct geographical areas, so basically you may have to maintain a central repository of tenants and, ultimately, under the hood, your tenant primary keys are not handled via local sequences.