Milan, Lake Como, Florence

If going to the Uffizi Gallery in Florence reserve tickets two-months out so you’re guaranteed a reservation. With six nights in IT…
Apr 6 - buy a train ticket from Milan’s Malpensa airport to Florence’s SM Novella station. This will require a transfer at Milan’s central station.
Apr 7 – Florence
Apr 8 – Florence
Apr 9 – take a direct train from Florence’s (Firenze) SM Novella station to Milano Centrale (2h): https://www.trenitalia.com/en.html. Sleep near the Milano Centrale train station where most of the sights are.
Apr 10 – take a direct train from Milano Centrale to Varenna on Lake Como (1h 15m). When you get off the train find the ferry that will take you across the lake to Bellagio and window shop along the lanes with steps. Take the ferry back to Varenna and walk along the path.
Apr 11 – Milan
Apr 12 – take a direct train from Milano Centrale to Malpensa airport.

Stick with your original plan

April 7 is Free First Sunday-museums will be super crowded
April 8 is Monday/museums will be closed

I'd go straight to Lake Como and spend at least two nights in this beautiful location, then to Florence, and Milan at the end. Just a suggestion......
Have a Great trip!

April 7 is Free First Sunday-museums will be super crowded

I'm assuming it's free for those who wait in the 4h line, not the ones who made advanced reservations. I know that the Uffizi never got crowded because only a certain number of people were allowed inside, the same with Accademia. It would be a shame if that were no longer true.

How do I get to Milan Porta Garibaldi from Bergamo Airport. Trains tickets on 6th April are sold out. Do they have good buses?

From what I can tell there are no direct busses from the Bergamo Airport Bus Station to Milano Porta Garibaldi. You will need to transfer at least once if not twice.