About the packing, shoes and everything else.
You said "We are going in mid-May to mid-June to Germany, France, Italy, Croatia, Austria, Slovenia, Switzerland, Lichtenstein..." and "We are mostly staying in smaller towns, usually in the old town areas."
Before you make too many decisions be sure to check
to find out what the weather might be at the time you will be at or near the places you are going. If you explore that website a bit you can get a few years of historical data for them.
That way you can better pinpoint the kind of clothes and shoes you need to wear while there. For example, the last thing you need is to depend on sneakers that won't dry quickly enough if they get wet.
I also have foot and ankle issues and one of my biggest shoe errors was wearing sneakers in Italy. They simply did not have the ankle support I needed for the uneven, broken pavement and cobblestones I experienced there.
Before that mistake and definitely after, I've worn ankle boots that are lightweight, supportive and comfortable enough to wear about 95% of the time. They are waterproof, good on paved flat surfaces, cobblestones of any shape, grass, gravel or dirt -- dry or wet. Far from fashionable, but I do have some fun colors.
In addition, I pack one pair of just-in-case comfortable flats that I rarely wear outside the place I'm staying and one pair of quick drying ballet style non-slip shower shoes that I bought after experiencing some very slippery shower and bathroom floors in 2018.
As for the clothes, I pack for a week and plan to do laundry along the way.
I may sink wash (my least favorite), self-launder where I'm staying or nearby (my most favorite), send it out or drop it off (both acceptable). The cost seems to work out to be about the same for almost everything I have except what I'm wearing.
My basic clothes list includes the shoes mentioned above and what follows. This is everything, including what I'm wearing.
On the inside: 2-3 pairs of Sockwell knee high moderate compression socks, 2-3 bras, 2-3 shapewear camis, 7-8 pairs of panties.
On the outside: 6-8 tops (including 1 button-up long-sleeve shirt), 3 pairs of pants and a rain jacket. The tops and pants are chosen based on historical weather data and the forecasts just before I leave. I might add a zip-up fleece or puffer jacket if needed.
Most importantly for me, everything has to go with everything else. I am not a fan of neutral colors and usually use a scarf to base my colors on, even if I don’t always wear the scarf.
I do carry-on only about 90% of the time. All my bags are 2-wheeled ones. The heaviest one has a U-shaped handle and weighs 5.5 pounds empty. The other 3 have T-shaped handles and weigh 4.5 pounds empty each. No matter which one I use, if it goes over 20 pounds packed, things are removed until it meets that goal. I use a backpack or a cross-body bag for my personal item.
And that's it. That formula (or less) in various permutations has worked for me on trips to Europe since 1977. And, yes, sometimes less is more.