@MatthiasWegner, I see you posted a follow-up thread Static Content Macro with Spring Boot, so I’m not sure if you still need any input here. I have a couple of comments though.
Avoid the Static Content Macro module if you can. It has major drawbacks compared to Dynamic Content Macros. Where the former require page-blocking server-to-server requests to obtain Confluence storage format from an app endpoint you provide, the latter simply render an iframe and a request is made from the user’s browser.
The Custom Content module mostly consists of configuration. There are a few references to UI extension points, but they are typically to the Page module.
For getting started with atlassian-connect-spring-boot and Cacheable app iframes, the approach we recommend for serving static HTML/CSS/JS as your modules, using the Spring Boot app as your backing API, have a look at [atlassian-connect-spring-boot-sample-cacheable-macro/](https://bitbucket.org/atlassian/atlassian-connect-spring-boot-samples/src/master/atlassian-connect-spring-boot-sample-cacheable-macro.