I've very little experience of the German military, but a friend (and one of the best Test Pilots I've ever flown with) lost his job with Eurofighter in Germany after failing his medical - as I recall as a result of back problems from a much earlier ejection from an F4, after many years of fast jet flying in the RAF since that ejection. Germany may be a little more strict medically than some other countries.
If you are shooting for Astronaut, Barnstormer, without doubt military TP is by far the most common route, and fast jet is preferred. But don't convince yourself it's the only route - the majority of the ESA Astronauts for example are in fact mission specialists, not pilot astronauts. Typically the background to such a person is:-
- Very good science or engineering degree
- Fair bit of flying experience
- Fluent English and/or French, conversational (minimum) in the other
- (Preferably) Doctorate in something experimental (not, ever, computing).
- (Preferably) Significant test flying experience.
- (Normally) aged 28-38 at application.
So, your approach, if you make it (and don't get too confident, for goodness sake) of an Aero-Eng degree + military (ideally test) flying is not going to do you any harm in that regard. But in the meantime, put 110% into both the degree and the military flying you are acccepted for, there is nothing "second best" about either career and the chances of being accepted by ESA are incredibly small, no matter how talented and fit you are.
ESA itself publishes a (quarterly?) newsletter on it's manned spaceflight activities. As a citizen of an ESA member state, you're entitled to it free and, although I've not looked for a while, I'm pretty certain you'll find details of how to subscribe on their website somewhere. Past issues have included a lot of information on Astronaut selection, training and recruitment, so you could do worse than find a library that keeps back copies.
And good luck with whichever career - there's nothing on your list of options that isn't rewarding and worthwhile, but probably nothing easy either !
G