I only have one year of German, and not a lot of language aptitude. In big places like Berlin, waiters always answer me in English when I order in German. Another language-practice idea besides Hostels (which don't appeal to me) is Zimmer Frei places. We can afford hotels, so I don't have actual experience there. I've seen a lot of Zimmer Frei signs while driving a car, which I don't want to recommend to you!
A post halfway back in this thread hinted at a fact we learned on our "Former East Germany" trip, which included an all-day drive out and back to Quedlinburg. (For the church treasury and town, not the language ... ) That is: On another trip, when my wife and I went to the Grunderzeit Museum in Mahlsdorf (suburb of Berlin), I was too late to book an English tour. I had to translate for my wife because our hostess said (in German, naturally), "When I was in school, the Foreign Language was Russian." This may be a clue to focus on major Ost locations (that have plenty to see and do), like Leipzig, Erfurt, Weimar, and similar. I was sorry to miss the Karl Zeiss museum (a specialty museum to be sure), but I'll bet some of the guides and guards don't like to speak English there. I wanted to go because of all the Zeiss planetarium projectors we used to have in the US, before Heavy Metal Nights required video planetarium installations!
By the way (not much German really needed ... ), I love Cologne. It is not true that the Cathedral (#1 tourist attendance numbers in the entire country, and some windows by Gerhard Richter ... and a great treasury) is the only thing to see. The "old town" is one of the weakest, but how about the Judenstrasse, the medieval mikveh, the Roman tunnels and excavations, the botanical garden, the Picasso and medieval museums ... I've been there twice, for a total of eight days. And that doesn't count next-door Duesseldorf:
The reason you want to go to D'dorf is because of the Longest Bar In the World (i.e. the Altstadt, not a single bar ...) You will be seated at a table of eight, already full with four to seven people. They will either HAVE to, or be GLAD to talk to you in German, every night. As I have stated elsewhere on this board, I was seated with three German chemical company duuudes, and the four Chinese middle-level guys who had come for days of training --- in English, I think. But all we spoke at the table was German, and while I bought my own food, my money was no good for alcohol.
Try to be in D'dorf for (Covid permitting) the one day monthly "Fish Market", which is really a riverside sausage, beer, and wine festival. Few fish are involved. You'll get plenty of German there. I also found little English in the D'dorf art museums. But one of the guards recognized me as a morning solo visitor when I came back the next day with the group tour I was there to meet up with! Nice folk.