As we now have completed our journey I like to give a quick update.

We had no struggles at all flying QR J to short check one bag on each way PLUS the stroller. Neither in SYD nor in FRA this was a big thing. It obviously took some time as the gate agent had to do it differently, but with our 2 kids they understood that we might need some additional stuff for our 20hrs layovers and they did a great job to accomondate our wishes. A really perfect experience (as was the whole flights)