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.