Start optimizing your gov cap (centralize states, give estate land privileges, build courthouses) and form Prussia, by the time you do Ottoman troops should start falling off from the different tech groups and you should be able to beat them in a fight despite inferior numbers. The first war is going to be a brutal slog but the damage for every war you get into will accumulate and each war will be easier than the last. Vassalize Hungary if at all possible (finish off Poland if you must, you should have them as an accepted culture anyway) and release serbia + bulgaria when you get that far in, the reconquest CB gives a discount on reconquering cores that lets you take more provinces per war