I'm an Egyptian married to a German, same-sex marriage. I cannot renew my passport at the embassy because I have reached the age of compulsory military. My residency ID in Germany expires with my passport, next year. And we cannot find a way to avoid the issue of me being forced to leave. It is dangerous for me to be admitted into the military in Egypt as a gay man or even return there. We don't know what to do. Is there anything we can do? We asked at the townhall if it is possible to have an ID with a validity that exceeds or not tied to the validity of my passport, but the answer we got was no.
I have never heard of being issued a shortened Aufenthaltstitel for Familienzusammenführung because your passport is not valid for the whole duration. That seems fishy.
However, that is not really your problem. Your problem is that you need a valid national passport from your home country at all times accompanying your residency permit. The residency permit, although it looks a lot like one, is no German ID and it does not replace the need for you to have an ID with you. Even if your residency title was for 3 years, you'd still be in trouble without your valid native ID.
I cannot say anything about how to get a new Egyptian passport.
Familienzusammenführung assumes you have a valid ID. If you cannot get one, you should definitely get a specialized lawyer.