I would suggest this book: Cloud Native Patterns (https://www.manning.com/books/cloud-native-patterns?query=Cl...)

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.