You will need an address in Germany with someone opening the mail for you.
As you state you do not have family or friends (close enough that you can rely on them) you may have to look for a paid service that does this.
I know that in some countries these services exist, that hold all mail and open those you indicate need opening or open all mail and forward ...
not retaining apartment
you are planning to return after a specific period
de-registering your address (you will not be returning to that address)
you ID will be amended with 'No main address in Germany'
This will stop all official post (which may contain any deadlines) from being ...