Does a possibility exist to setup a restricted access, so that customer are able to have an insight in tracked times of specific projects?

Yes, a restricted access is basically possible. Therefore you just setup an employee account. Don't tag this employee with a role, so that he can't insert or delete data. Afterwards you just assign him the projects he should be able to see and activate the checkbox project reporter. This employee now can only see tracked times of this project in the evaluation section.