I would look at using a cargo carrier on the roof and keeping the spare in there, as you might get pulled over if driving down the interstate with a spare tire strapped to the roof. And the mpg loss would only be on a trip which is probably the only time they would do this, when closer to home which is probably 99% of the time they could keep the spare in the trunk, or keep it at home and use Uber to go home and get it. It's when you are hundreds of miles away from home and have a flat tire that things can get very unpleasant. Even if you don't change the tire yourself and call AAA or whoever, at least if you have a spare they can put it on for you and you are on your way. But with no spare you will have to be towed somewhere, so depending on time of day where to? Maybe a tire shop and hopefully they can fix the flat, but if not and you need a new tire it is unlikely that the tire shop will have our quirky OEM tire in stock. If you are lucky maybe the dealer does, so then I guess now you see if they can deliver it, or Uber to the dealer to get it and bring it back to the tire shop, or just have the car towed from the tire shop to the dealer. Or maybe get towed to a dealer first if one is nearby and their service department is open, increasing the chances that the first place that you get towed to has a tire in stock in case the flat cannot be fixed. And also from what I hear tow truck drivers are not currently allowing you to ride in the cab with them, in which case you would wind up having to Uber yourself to wherever the car is being towed to. I realize I am painting a worst case scenario but it is by no means far fetched.
Losing a few mpg seems like a decent tradeoff in comparison if storage space in the trunk is an issue.
A donut spare would take up less space in the trunk, and at least allows you to drive to a tire repair shop or dealer. Although if they don't have the OEM tire in stock and it takes two days to order then you are in the same boat because you might be too far from home to safely drive hundreds of miles at highway speeds on a donut spare.
Another option somewhere in the middle, although I have never heard of anyone doing this, would be to purchase a used tire the same size as the OEM tire, but don't mount it on a wheel just carry the tire in the trunk. You would still have to be towed to a tire repair shop but at least if they can't fix the flat you would have a tire that they can mount for you. And also keeping just a tire in the trunk would give you a little bit of storage space in the middle of the tire, or being creative you could even stuff things into the tire itself.