Are you really cycling far enough to warrant a flight? Is it about the journey or the destination? I would think you don’t do that kind of distance without loving the journey to the point of trivializing the destination. So for a given trip, you could just as well cut the outbound trip in half. It need not be a back and forth trip as the redundancy is what you want to avoid. So make it a triangular route.

Whatever your distaste is for buses, perhaps that can be remedied with small mid-trip stretches. Find a city pair where the journey is overall uphill, then book a bus just for a short segment of your trip. You get a more interesting window view for an hour or two than you would in a plane, and a moment to rest between cycling segments. You can do the same with ferries.

It’s also surprising to hear that you find it easier to take a bicycle on a plane than a train or bus. The fee for that must be small on buses and trains than on a plane. And hard to trust airline baggage handlers with your bicycle.

Might be worth looking into taking a blabla car, though it might be hard to find a ride who can spare the space for a bike.