Ok first of all...
Under FAA rules and regulations there is no requirement you fly an N-reg.
So you can fly a German registered airplane with a dual rated instructor ( EASA/FAA CFI) for your flight review.
As long as you are within 24 months of your last flight review or if you’ve never had own your last check ride w/examiner you can log PIC time as ‘sole manipulator of the controls’ and it’s an airplane you’re actually rated for....such as fixed gear single engine.
There should be a bunch of FAA CFI’s in Germany.