Based on Simon Brown's talk about modular monoliths - I tried to apply something simulair myself and ..... this is what I came up with. See: https://www.youtube.com/watch?v=5OjqD-ow8GE
Besides that I implemented the different layers of Spring Boot tests in a way that I feel it should be done. See: https://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-testing.html