You asked about your time allotment of 3 full days in each of your cities. Although you could definitely spend more time than that in each place, I think it's reasonable for a lot of your targets (not all).
Munich should be OK, even with a day trip; Munich isn't blessed with a lot of sights most people consider must-sees.
Three full days is OK for Venice (not a large place).
It's probably OK for Amsterdam and Florence if you aren't art nuts who have to go to all the museums.
Three days is quite short for Rome unless you're certain you have limited interest in the historical sights from the Roman period and don't want to see many churches or art museums. In other words, three days to wander around some of Rome's neighborhoods and gaze at monuments from outside wouldn't necessarily be too hectic. But the city's on your want-to-visit list, so I bet you have an interest in Roman ruins, churches and/or art.
Three days probably won't cut it in Paris or London unless you're most unusual travelers.
The above are just my comments, which are no more valid than anyone else's. If you haven't done so, I'd suggest reviewing Rick's sightseeing highlights for the cities on your itinerary to see how many seem to fall in the "gotta do it" category for you. Rick doesn't list every sight in a city, but he has pretty good instincts about what appeals to most American tourists.
Rick's top sightseeing priorities can be found right on this website:
Choose Explore Europe (in the left panel if you're using a PC)
Select the appropriate country.
Select the city you're visiting.
Click on At a Glance.
One of our other posters has suggested expecting to hit two major sights per day. I've found that a good rule of thumb and a decent guide for guesstimating how many days I need in a city. It usually allows some slack for wandering around the historic part of town (which is very important to me), seeing something I learn about after arrival in town or relaxing if I've been pushing too hard. Folks who are serious foodies would also appreciate having time for leisurely meals.
Trips such as you've sketched out can work, as can deep dives into one or two countries. I think it's very desirable to have different types of destinations when you have a trip of 2 weeks or longer, but variety can be built in in several different ways; it doesn't have to mean a large number of countries, or even more than one. You can include smaller cities as well as large ones, spend some time in rural areas, see mountains/lakes/coastline, go to culturally-different regions of the same country (as you're doing with Rome-Florence-Venice), indulge in activities you enjoy that go beyond regular sightseeing (athletic activities, cooking classes, etc.) and so on.
I'd suggest you consider whether so much time spent in cities that are all very touristy and mostly very large will be somewhat tiring on a trip of this length. Smaller places are often more relaxing to visit because you don't usually have the pressure of a long list of sights you plan to see. Typically, smaller cities are also much less likely to be infested with a bunch of Starbucks, Pizza Huts, McDonalds, KFCs and so on.