12 Days In Italy-Appreciate Experienced Traveler Opinions

Congrats on the flights- what a score!
Six nights in Milan would be way too much for me, as it's a bustling city. Seems you have free Marriott lodgings, but you'll be missing the serene and beautiful mornings and evenings of staying in smaller towns. Would you consider a few nights in Bologna?
Just curious, what are your interests in Genoa?
The RS guide has some really good recommendations for Milan and Florence. Also, check out the Trip Reports here, and the Scrapbooks.
I'll do some Searches here and see if I can find more info regarding your questions. Enjoy your trip!

https://search.ricksteves.com/?button=&date_range=2y&filter=Travel+Forum&query=Florence&utf8=%E2%9C%93

https://search.ricksteves.com/?button=&date_range=2y&filter=Travel+Forum&query=Genoa&utf8=%E2%9C%93

https://search.ricksteves.com/?button=&date_range=2y&filter=Travel+Forum&query=Milan+restaurants+&utf8=%E2%9C%93

https://search.ricksteves.com/?button=&date_range=2y&filter=Travel+Forum&query=Florence+restaurants&utf8=%E2%9C%93

We spent 6 days in Milano in March of 2020.
Enjoyed it, except for Zona Rosa. Covid was beyond annoying.
We are returning this October for 4 days. Maybe The Last Supper will be open.
Climb to the top of the Duomo.
My wife loved the Bottega Veneta store where she was the only customer for that week. It was an expensive visit.
We ate well.

If I were to do a daytrip to Bologna I would do it from Florence, not Milan. Six nights in Milan would be too many for me but I prefer smaller towns to big cities. Since you are interested in Bologna and food would you consider a two night stay there? A pretty daytrip from Milan to Bergamo is doable or to Brescia. I'd take some of those nights from Milan and consider adding in one more destination, or adding days to your base of Florence. Some of the other destinations to consider for a stay or daytrip from Florence would be Verona, Siena, San Gimignano, Mantova.
Sirmione has a very pretty castle on the water on Lake Garda. I really liked Riva del Garda and Limone sul Garda on the northern end of the lake, nestled among the mountains. You would need to get there by ferry or car as there is no train service to those two scenic towns. Malcesine was also attractive. I visited them on a daytrip by car with a driver service from Trento. To get to Sirmione I took a bus from Verona. The town of Sirmione is very small.
Should you decide to stay a few nights on the coast near Genoa look at Comogli or Portofino or Santa Margherita Ligure. I, personally, would avoid Cinque Terre as I imagine it will be packed with tourists in July and August.

Great deal on your 2 for 1 flight! Enjoy your trip!

Considering the weather in the last two years, the temperature in Milan in early August may be a bit high. The maximum temperature may exceed 30 degrees Celsius, the ultraviolet index is relatively high, and there may be very sudden thunderstorms. To be on the safe side, you need to prepare sunscreen products and an umbrella.

If you want to visit Bologna, you’re only a 30-minute train from Florence. Would you be okay with 3 nights each in Florence, Bologna, Lake Garda, Milan, instead? If so, take a day while staying in Bologna to go see the gorgeous Ravenna mosaics. It’s an easy train ride.

From Milan, you are near several places within an hour by train: the lakes - Stresa on Lake Maggoire, Lake Como, Cremona - very atmospheric piazza with an amazing fresco-filled cathedral interior. Plus Cremona is home to Stradavari and an excellent violin museum w/ concerts on weekends. Torino is even just an hour by train and lots of history - a great Egyptian museum! And more.

A small bit of advice. Use your Marriott points when it’s to your advantage. But also consider where the hotels are located compared with the ideal location for sites you decide you want to see. Sometimes it becomes a hassle if you’re going out of the main center to get back to a hotel, taking away precious time. And don’t eat at the hotel; so many wonderful local restaurants . : )

Have a great time!

It may indeed be very hot in Milan in August, but Florence will almost certainly be noticeably worse. It's difficult to avoid uncomfortable summer heat you're in Italy south of the Alps; just be mentally prepared for it. I'm sure you can count on good air conditioning in hotels of Marriott's class.