My wife and I rode through some of the country you are interested in as part of a longer tour. Is this your first tour? If you end up in Croatia, how are you getting home from there? With only 4 weeks I believe your route is a little ambitious, especially if you want your GF to have a good time, and ever come on another tour with you.
A nice ride of about 1000 miles would give you time to see some of the places that are worth stopping at: day in Berlin, some of the neat towns in Poland, and Prague. It would be relatively easy to set up an arrival in Berlin, and a departure from Frankfurt.
Our route from Berlin went east to Poznan, PL , south to Wroclaw, PL; east to Prague, CZ; east to Cheb, CZ; and then east into Germany picking up the White Main River at Bayreuth. There is a bike route along the river into Frankfurt. We have used the Frankfurt Airport before, and while it is not especially bike friendly, they do sell bike boxes. However, they are in a separate terminal form the airline we were using. It pays to get to an strange airport a day or two before the flight to figure out the logistics. If you get to Frankfurt with a few days to spare, a nice ride would be up the east side of the Rhine River to Koblenz.
I really like Poland, and if conditions permit, I'd like to back there someday. We started in Denmark, and ended in Amsterdam via of Germany, Poland, Czech Republic, and Belgium.
Good luck on your ride, and hoping things will be a better by next summer.
Last edited by Doug64; 09-29-20 at 11:05 AM.