Judging by the rigs most full-timers use, I'd say a Bigfoot
is too small for a couple.
Clearly this family is not following the crowd. They're already full-timing in a tent trailer!
Unfortunately I can't think of any molded fiberglass trailers that check all the boxes: 6 beds, bathroom, galley, under 5000 pounds, van-towable.
I think they need to be careful about total weight
here, too. With 6 people in the van and, apparently, a lot of cargo, they may exceed the van's GVWR or GCWR if they get anywhere near a 5000 pound trailer. That 5000 pound rating typically assumes only two people and minimal cargo in the vehicle.
As you say, some hard choices lie ahead. But I wish them well!