Decomposing the system into libraries is just as effective from this perspective as decomposing into services however. The thing that a typical http API gives you is stability (documentation, versioning and a deprecation policy). This can be done with a library as well. We know this works as 3rd party libraries work well and no communication with the authors of the library is needed.

This is why I suggest team size is not a valid reason to break a problem into microservices. The problem itself is the thing to analyze.