Comment by jeffrallen

One nice compromise is to migrate to using read-only database connections for read tasks from the moment you upgrade from medium sized DB hardware to big hardware. Keep talking to the one big DB with both connections.

Then when you are looking at the cost of upgrading from big DB hardware to huge DB hardware, you've got another option available to compare cost-wise: a RW main instance and one more read-only replicas, where your monolith talks to both: read/write to the master and read-only to the replicas via a load balancer.