Fair and valid points made by LOMCEVAK.
A TP needs a high level of both flying ability and experience, and high engineering knowledge and ability. There's a small trade-off there, but only a small one.
Also, realistically, the TPs with an FTE background who have built their own flying experience will mostly never tackle the biggest and most complex test flying tasks as a TP. Given the size of the helicopter world in particular, that is extremely limiting. Most of us with that profile are part time, and exist in the light aviation world, not "heavy metal".
I believe that there may be a couple of engineer-background TPs at, for example, Robinsons - but the odds of landing such a job are incredibly small. And doubtless those TPs are as high ability as they should be for such a critical role.
If the OP is of an age and profile who could potentially join the German armed forces, that is almost certainly his best route if it's available. Most armed services these days are very amenable to good people pursuing additional technical qualifications alongside their main service roles, and Germany does put people through TPS - albeit few, and they both need to be very good, and have the right profile at the right time.
G