Slovenia & Italy in one trip

That would be too rushed for me. to evaluate for you and your pal, write it out day by day and mull over how it looks. Don’t forget to include travel logistics.
Multi city flights can certainly be a good idea, check all iterations on google flights.

You've listed 8 places and want to do it in 14 days, including transit. Do the math.
Unless you can make 3 or 4 of those places day trips, which I cannot assume for you
since it really depends on your interests, you will most likely spend about 36 hours
in each place. You will go to many cities and not see much of anything.

You're falling into the same trap we all do - trying to fit to fit a square peg of travel
into a round hole of schedule.

At the least, cut out Milan. The rest of your destinations are geographically within
earshot of each other and could be managed with a rental car. And dispose of any
notions of Florence and Rome, unless you can add another 2 weeks into your plans.

As far as flights, knowing where you'd fly from is important to answering that question.
But while a multi-city plan is just fine, you can also look at the Italian and Slovenian
sections as 2 parts of a figure 8, and make, say, Trieste, the intersection and thus the
arrival airport.

Yeah, you just need to start writing this itinerary out and make some priorities. My hot itinerary: 4 nights Venice, 2 nights Trieste, 3 nights Piran, 4 nights Ljubljana (daytrip Bled.)

S,F, Solo, 78. I usually travel 2x a year, 4-5 weeks at a time. Did Northern Italy (4 weeks) and am doing Italy and Slovenia April 5-May 12, 2025 or in a few weeks. So my time frames are different than what yours will be so basically for info purposes only.

PHX to Rome, Ljubljana to PHX. Rome (week, and 6th trip), 2 nights Perugia (2nd trip), Florence (spent 1 month in 2009) one week in April, side tracking to San Marino (this is solely to complete Smallest Countries of Europe list), Vicenza and Venice for a couple of days each but have been there in the past couple of years. Onto Piran (a cruise stop in the past) and the :Ljubljana for about 5 days with a day trip to Lake Bled.

I always use booking.com, 95% of the time rent apartments and usually with washing machines. I only bring a carryon and a satchel so washer almost a necessity and I like a microwave more than a stove or oven. I did not find anything worth stopping for in Trieste and am bypassing it again except to switch modes of transportation from Venice to Piran. I find Florence INCREDIBLE and Venice is certainly unique. Florence site days/hours are the stuff nightmares are made of. One site I am lucky enough to be able to see is only open on the 3nd Monday of the month from 3-6pm! Yikes! So if you add Florence make sure you can see what you would like when you are there. Regards.