looking to set maxMessageSizeBytes option as mentioned here in a more declarative way (i.e. via Helm), it doesn't seem clear how to do this. Any pointers?

You could update the chart value to the below

server:
      grpcPort: 8089
      httpPort: 8088
      grpc:
        maxMessageSizeBytes: 4194304
      security:
      ...

Perfect, thanks!