I'm not the OP and I drive a large SUV, but I would consider a minivan in the future if the price was right. The primary metric for me is that I'm 6'7" 300 and I need something that is comfortable to drive. From the driver's position, the Sienna is most comfortable for me.Oh OK, then drive the mini until the wheels fall off, and get you another one. It really comes down to whether YOU think the minivan is worth to fix or buy another that you like more.............I will give an example......
About 9 years ago, I was in the market for a 3\4 ton Suburban. I found what would be probably in the top 1% in condition, a 99 K2500 7.4 4x4 SLT. At the time, used cars were not as ridiculous as they are now......anyway I paid about 5000 over book for the truck becasue of its condition, records, and the like.......did I screw myself with paying a higher price? Well, at the time, a new one (2014) in the same trim was about $60000 and had a few creature comforts that my dinosaur GMT400 burb did not have, but a sixth of the price..........I still have the Burb to this day, and will likely never sell it, runs like a top, an without most of the BS in todays trucks. Did I get a good deal, or make the right choice, maybe.
If efficiency is the name of the game, and the size of the passengers dont matter, trade in the minivan and get a civic. If comfort and space plus efficiency is the name of the game............who knows.
Assuming the van is in good working order, and you OWN it, I suppose the "efficiency" has more to do with cost of operation than MPG.........bunch to decide. All and all, the Japanese makers you mentioned in your OP are "better" as far a reviews are concerned, but that is subjective I suppose.
The total cost I think is what efficiency is, not just the MPG. Is it worht buying a new van for 50K if you own the one you are in? NO! You can buy a bunch of gas for 50k..................sorry this might not be what you are looking for, but just my 2 cents.
Also not hating on minivan ownership, though I think that they have a niche and to me are about worthless in the grand scheme of things.
I have ridden in the 2nd row of a minivan in the recent past and it was fine. The reason for my response was your quip that you need a large SUV if you have larger kids. If the 6'7" guy can sit in the 2nd row of a minivan then it's certainly comfortable enough for bigger kids.
For the time being I have made the same calculation as you, i.e., I can buy a lot of gas for what a good used minivan costs. When I wear the wheels off the Navi, the answer may be different than it was this time. 10 year old Expeditions and Navigators are back to being cheap so it's hard to argue for a minivan over those. I'm seeing them under 10K a lot now. What would make the difference is if I could get a hybrid minivan for a good price, I've seen the Uber drivers with those getting 35MPG. Almost three times the mileage might sway me vs large SUV mileage, to up my used car budget. I could care less about towing, as I never do.
I would never consider a used suburban with a big block for a daily driver, even if it only had 10 miles off the factory floor and it was free, but to each their own.
I would definitely like to have a squarebody big block Suburban as a weekend vehicle, but that's a different story.