Hi Uli,
Thank you, that sounds very promising and I am happily looking forward to your contribution!

• Regarding the PMode, as this is an EU funded project, I assume the "CEF" based components are good starting points.
• For the client, the cef and the bdew clients are pretty thin layers and just add some common functionality, where the peppol-client has a lot more business logic in it (like support for validation, SMP lookup etc.). As I don't know the details of the ICS2 profile yet, I cannot make a good judgement call. As I am in doubt, I would recommend to start small and look at the cef client.
• For the server there are 2 different approaches: as phase4 is in general "thirdparty framework agnostic" and therefore runs without Spring and without the Servlet specification as well, I see the Spring Boot stuff really just as a demo. However, as I also saw people struggling with the multi module Maven project usage, I extracted a totally separate project https://github.com/phax/phase4-peppol-standalone that people can use as a kick-start. However, that project of course has less visibility compared to the main project.

So hope that answers your question.

Additionally, I sent you an invite to my Slack channel (phoss.slack.com) and encourage other interested parties to join as well - that also worked pretty okay for the BDEW implementation etc.

hth, Philip