If your flight is 10:30pm, then you'll need to be at the airport for 7:30pm to 8pm allowing time for

Thanks for the good advice on here and response to my questions. I've booked all our accommodation - yippee! Now I'm looking at the best options for car hire for our last 7 days. We are leaving Rome 3 July, booked into accommodation on Lake Garda for 7 days until 10 July, then fly out of Milan 10:30 pm that night. The nearest train station to our accommodation is Desenzano.
Would the best thing be to pick up a car at somewhere like Orvieto, and drive to Lake Garda, then return the car maybe to Verona and get the train to Milan Centrale? Or train Rome to Milan, train to Lake Garda/Verona and pick up/return car there? I've suggested Verona as an option because lots of car hire places seem to have offices there, but would look at whatever people recommend.
There will be five of us and when I look at the Trenitalia websites the train tickets out of Rome to that area seem very expensive. airline minute airfare ticket fare We are paying for a car for a week anyway, but I know fuel is expensive so would it work out much the same. On Trenitalia Rome - Verona trips are coming up around 95 euros each per adult.
You should get rental quotes for different pick-up/drop-off scenarios and compare to estimate driving costs you can get from viamichelin.com . You're going to need a fairly large car to hold 5 people and all the luggage.
If there are 5 of you plus luggage, airline minute airfare ticket fare then you'll need a big car. However, the autostrada airline minute airfare ticket fare runs to Milan Malpensa and you pick it up quite near the Desenzano area. So hire your car in Rome and head North to Lake Garda, keep the car until you have to return to the airport and turn it in there.
If your flight is 10:30pm, then you'll need to be at the airport for 7:30pm to 8pm allowing time for the car return airline minute airfare ticket fare and check in. If you leave Desenzano after breakfast, you'll have time to stop at Bergamo for lunch and sightseeing and then on to Milan. The autostrada can get very busy depending on the day you travel, so leave plenty of time to allow for hold ups.
Thanks for the advice - I think we will probably do the train as its about half the distance and pick up the car in the north. For us Australians driving in Europe is scary because not only is it manual but its also on the opposite side of the road. Last time I did it I refused to go near the autostradas which made for a slow but probably more scenic trip!