A successful investment requires relationships and knowledge of what one is investing as well as the factors involved. Relationships take a long time to build, perhaps longer in Germany than in many other places. Knowledge of the investment and the factors involved is all about experience. You can obtain an instant picture of the price of an investment property at a given address, for example, but it requires experience and detailed knowledge of the precise geographical area to know why a return of X% is not good enough, even though that would be more than average.
There are countless parameters at play when an investment is to be made, and it is impossible to carry it out successfully without a network and knowledge of all the factors.
This is why you need a partner and advisor for your investments in Germany.