Agreed - no need for the stress! If Berlin is your base, you may be happiest just staying there. Like hamburg, it's huge and easy to spend your whole visit there.
I used to spend 10 days (in February) a year in nuernberg for a trade fair, and loved it. I think it's easy to stay in one German big place like Berlin hamburg et al for an entire visit plus in Winter, it'll be cold and you'll want to have time for long coffee & cake breaks.
Happy travels and may there be very low stress :-)