It expresses so well the problem faced by OP and how they should be tackled. Sure, there's no proper rule of thumb, since every project is unique, but a good architecture should address these concerns
> As you can probably guess from the high service-developer ratio, we ended up with lots of “separated” yet tightly-coupled services. This hurt us a lot.
This isn't related to Conway law. This is just the team ignoring key design principles, or lacking the discipline to follow these principles.
If you have a small team it is unavoidable that people will work on, or own, several microservices. This does not mean that coupling should increase.
It expresses so well the problem faced by OP and how they should be tackled. Sure, there's no proper rule of thumb, since every project is unique, but a good architecture should address these concerns