It looks like you're dressing for warm weather and you probably have thoroughly checked out the weather in the places you're going BUT I just want to mention that I went to Europe at the end of April and was freezing in some places! (Switzerland and Germany and even Paris was rainy and cold!) Again, I know you've probably already done the research but for the benefit of future searches I thought I'd mention that!
For shoes, if it is truly going to be very warm, I cannot recommend anything more highly than Fitflops. They're my go to for Disneyland and fortunately the end of my Europe trip last year we were in Nice and I could wear them! I have a tricky knee and last year I sprained my ankle and the Fitflops were as supportive as I needed with those prior injuries. I would try out the sneakers you already have and see how your feet feel after a very long day in them. I think that buying new shoes can be a mistake for a long trip. I also see the advantage of bringing multiple pair. I wore several pair of shoes that I felt were "well tested" and yet after a full day, there were PARTS of my feet that were sore! Like my tall boots had somewhat thin soles that didn't like a whole day of walking on gravel and cobblestones, and my waterproof wedge booties didn't like consecutive days spent entirely on my feet! (I will note that I never have a problem with the Fitflops, but they don't work for cold and rainy!)
Clothes wise, this will vary based on your taste and style. I'm such a fan of wearing dresses in Europe. Especially if it is warm! IMO they are as comfortable as any shorts/pants options but they feel fancy enough for any potential destination--from fancy restaurants to cathedrals! Plus they pack up so small. I love a few dresses that I've recently purchased from Amazon. I'll link them here.
https://www.amazon.com/MEROKEETY-Wom...%2C141&sr=8-30
LOVE this one. Have it in 2 colors.
https://www.amazon.com/ZESICA-Womens...zcF9hdGY&psc=1
And I have four patterns in this style, lol.
Hope this helps!