Do you folk have any tips about bicycle routes in central Europe ? Are there remarkable things you dearly remember or places/routes you wish you didn’t go though ?

My gf and I are planning our summer trip, we are going from Rheinland Pfalz in Germany to Transylvania in Romania and then in Dalmatia in Croatia and back home via a boat then through Italy. It’s our first very long trip, we only did a 3 weeks long route last summer, but this time we have 4 months.