In my trade, people usually only resign after they have another job offer (as in contract in their hands). I guess if I was in Germany it would circumvent this reference system at least for the current employer?
Resigning only with a new job secured is also common in Germany.
That's what interim reports are for (that employees have just the same right to as for the "real" reference letters), although they may tip off the current employer.
As others have noted, there are interim letters that you can request without reason at any time. But, there are also events that allow you to request them due to that event. For example if your team lead switches jobs, you can request an interim letter due to that, and it will be noted that this is the reason for writing it.
Those do not tip your hand and you requesting them is considered absolutely reasonable - how would your new boss of 3 months write that letter for your entire employment? These are what you use while still employed.