Type of rental car for Dolomites/Alps?

I am a car enthusiast

I don't believe that the BMW 4 Series Coupe is too large for your trip. You will see plenty of BMW and Mercedes sedans on the road and in town.

As a car enthusiast you’d probably prefer a manual transmission. If there are only the two of you a BMW 1 or Audi A3 would be good I just spent a week in the Dolomites in a manual Audi A3 (from Europcar) and I thought it ideal. Any chance Sixt can “downgrade” you to a smaller “premium” car?

I’m doing a similar trip out of Milan to Switzerland and then picking up a Mercedes in Italy for the Dolomites. I like your option 2 with the BMW Coupe.
If you lock your belongings out of sight in the trunk, I don’t think you need to worry about car burglars or thieves in the Dolomites. It’s affluent northern Italy and there are just too many other people around as a deterrent.
Have a great trip!

Locauto used to have a good reputation. Now it operates as a franchisee of Enterprise. Not sure how good it is, but if you check the reviews of anything you will find complaints about any company.

The decision is really between two different classes of cars. One is a luxury sedan (BMW 4 series) and the other is a compact (VW GOLF). The latter, in the TDI diesel version has the same displacement (2.0 L) and similar torque, so you would still have enough power to have fun in the mountains, however VW Golf in Europe comes also with less powerful versions (less torque and smaller displacement), so which one are you going to get?

In terms of car size, I wouldn’t worry too much. I usually rent station wagons or SUVs in Italy. A few years ago in the Dolomites I had a very long Diesel VW Passat Station wagon TDI, which had a 2L engine displacement. Diesel has more torque, so it was wonderful zipping through those roads with that car. Negotiating those roads with that size of car will be no problem at all. Actually this past month I had a Diesel Ford Focus station wagon which I used in Tuscany, Abruzzo, Marche, Puglia in even narrower roads and town streets that you will experience in the Dolomites, and had no trouble at all. Obviously it requires some skill to park in some instances, so if you are the type who has a hard time parking a car in a 2 car garage, you might have some challenges parking in some parking lots or in the streets, but I don’t remember having difficulty in the Dolomites where towns have larger streets than in other parts of Italy.

I drive in Italy every year and every year I notice that Italians have larger and larger cars. The days of small subcompact hatchback only are gone. You see smaller cars in big cities p, as people buy smaller second cars for daily commute in the city, but for out of town travel you see also a lot of big cars, Large station Wagons and SUVs (including very fancy brands) have become very popular and they are the majority of cars on the road now, even in the south, which supposedly have lower average income (I suspect they just cheat the taxman more and report less income, but I saw lots of fancy cars in Puglia this month)., therefore I wouldn’t worry about garnering attention. In the Dolomites you will see many cars that are far more luxurious than yours.

As a car enthusiast you might want to drool over these:
https://www.italianferrarirental.com/ A little pricey though Brad

Here’s a good site to compare car dimensions. Sometimes the differences are suprisingly less than you had thought.

My husband and I did somewhat of a similar trip through the Alps for our honeymoon, but did a round trip rental from MXP (Chamonix, Interlaken region, Innsbruck region, Bolzano/Dolomites and Lago di Garda). My husband really wanted a manual car to practice his skills again, and so that's what we did. We ended up with a very small hatchback that had just enough room for the two of us and our luggage in the back seat/trunk. The smaller the car, the easier it is to maneuver about on the VERY sketchy switchback turns along the drives. He had a blast driving along there - no need to fret. Those that were driving the larger cars (even what a Sedan is) had a hard time staying within the confines of their side of the road most of the time. It's difficult enough when a bus is coming your way.

Also, thieves are really not any more of an issue than they are in America. Just don't keep your precious belongings in eye's view or on your person. As long as you're smart, you shouldn't have any issues. We left our stuff in the back seat when we were out to eat on travel days but never had any problems.

Have fun!