This may or may not be that hard, depending on your server. You could proxy all "mutation" HTTP verbs to your one writer instance, and probably do similar if you are using GraphQL.
If you are using something like gRPC I feel this might be more complicated because it's not obvious which actions are read/write.
I'm in the same boat as you though overall - I'm not sure what the ideal strategy is, or if one even exists, since this seems to create a problem that does not normally exist.
If you are greenfield, maybe you create one service that only does writes - this may be CQRS-like.