I would like to point out that the old parts of many European cities, and many rural areas may not be comfortable for those with mobility issues. Sidewalks may be cobblestone, entryways are often step(s) up or down from the street level, elevators are lacking in many older buildings and for those totally wheelchair dependent, doorways may be too narrow. Also, not all trains, busses and trams can accommodate a wheel chair.

So, since you are planning well in advance, you have plenty of time to get the details you need about any place you would like to visit.
And, you have plenty of time to work on improving your mobility, so perhaps you will be able to use just a cane and/or walker at your destination, with wheelchair assist in airports and maybe train stations.

I would love to be planning such a trip! We are looking forward to 2025 or 2026 and a 3-generation trip to visit 3 generations of family in Southern Europe.
Sue