Depending on the year, the only part of the trip that might be dicey real early are the 2 nights in Switzerland. When I went on the 21 day tour (end of August ) the guide was saying that in June he'd had to cut out some steps in the snow on a trail in Wengen so his group could do a hike. That was 9 years ago so temps may have changed and snow may not linger that long...or it could have been an unusual year. I'd not recommend going that time in August - WAY too hot for this gal!
All other things being considered I'd pick the B date. If this is for 2024, Paris is hosting the Olympics the end of July but I'm guessing you should be OK in June.
I'd also try to arrive in Amsterdam a few days early and if you have the time and budget, stay in Paris for a few days extra.