Northern Italy in late April/Early May?

Weather unpredictable on Dolomites. Got caught in snow once in May. So be prepared.

April 25 and May 1 are national holidays (Liberation Day and International Labor Day respectively). It's probably the busiest time of the year in many places, it's a very long weekend for the Italians. So expect crowds at Cinque Terre and elsewhere.

Go to Varenna upon landing, assuming you don't land too late in the evening. Leave Milan nights at the end so you don't need to split Milan stay in two separate times. That might give you an extra night to allocate where you like.

I'm not opining on the number of nights in each place, because that is a personal preference.

Cinque Terre- 3 N ? Consider 2.
We have experienced the 4/25 national holiday so be prepared.

Thank you both! Didn't know about the National holidays. We are flexible with our travel dates and will consider travel later in May.

Milan - Como - Castlerotto - Parma - Cinque Terra - Milan

Put all your Milan nights at end of trip- go right to your location in Como on arrival so you avoid that split stay.

You might want to take a look at this similar thread that discusses the amount of travel time you are looking at:
https://community.ricksteves.com/travel-forum/italy/itinerary-recommendations-4d1be414-d120-471d-aa36-8bc18d9b224e

CT has become so over-visited that it makes me weary thinking of it. I would extend a stay in Parma, or better yet, Modena, which would allow you to day trip to a wider area and less train time.