read replicas with legacy code that doesn't properly separate queries

I was working on read-write separation (Aurora 4 MySQL) for quite some time and as far as I can tell read replicas do not have such an option.

If the implementation of query routing is not an option, then you may consider using third-party RW splitters, like ProxySQL, Apache Sharding Sphere (https://shardingsphere.apache.org/document/current/en/features/read-write-split/) or
Heimdall proxy (https://aws.amazon.com/blogs/apn/using-the-heimdall-proxy-to-split-reads-and-writes-for-amazon-aurora-and-amazon-rds/).

HTH

Excellent options, thanks!