I was coming from your perspective when I originally ordered the Y. Now that I got the hitch on my 3, I just through a carrier on it and problem solved for me. I have tons of room when I need it but I am not stuck with a vehicle that is less enjoyable for the other 90% of the time when I actually need the extra capacity.We have two car seats right now and a dog. Don't get me wrong, the cargo area in the 3 in huge, I'm not discounting it. I've taken my in-laws from the airport with 3 full sized suit cases and a roller bag. It really comes down to comfort with the kids and all the crap they bring with them. The efficiency thing does have me curious though. I'm going to plan some trips we do in a Y on ABRP and see how it fairs. I know it's going to be less range but it'll be interesting to see how that changes our trips, we rarely make it the full 180-200 miles I can do between optimized charges in the 3 anyway.
If you need the capacity all the time, the Y makes a lot of sense. For some people it is the best option. You also have the 7 seat option too but then cargo space is nil and you'd need a hitch and a carrier for even the Y. For others, adding a hitch on the 3 and then a carrier might be a better way to go.
I was all ready to love the Y until I lived with it. All the noise from the rear (open area is like a boombox) compared to the 3. About 1" less legroom compared to the 3 but you gain in headroom. I also found with the legroom in the Y makes my feet feel more scrunched together in the footwell than on the 3. Rarely have I been so happy to see a car go as when I sold my Y. Nor did I expect the 3 to be so much more fun to drive on a daily basis than the Y. I am still amazed at how good of a daily driver it is.
I'd say rent a Y on Turo for a week and see what you think.