Is it reasonable to leave a PhD if your supervisor leaves the university?
First thing: There is no such thing as "PhD transfer". (At least in Germany.)

I don't know your university, but is the cases I know, you do not start any official PhD in the first year, but only later. Usually, you make an agreements with some advisor about the PhD thesis, working contracts and so on, and sometimes you enroll as a PhD student, but in most cases there is no official file at the university (but this may change in the near future…). At some point (where I am, usually in the last year) you fill in some form with which you start the official process of graduating (Eröffnung des Promotionsverfahrens or something similar).

So, if you haven't started the "Promotionsvorhaben" formally, you can quit and start new at any point.

More practically: You should discuss your options with your advisor. She is the right person for this. She may have suggestions how to proceed, e.g. she knows people who would like to take you as a student, she has a direct line to these people and will get a response quickly. She may help you to judge the possibilities that you have at the university where you are and she may also know you, your strength, and your preferences already.

Finally, I find the notion of "important university" a bit odd. There are some universities in Germany with a bit more overall prestige but there also also many not-so-well-known universities which are top at specific fields. So, the university name does not mean too much.