Finally convinced my 7 yr old to ride his own bike down the hill out of the neighborhood (he would only do it on the trail-a-bike before) to go downtown to the farmers' market and some other errands:
This frees me up to pull our daughter in the trailer and my wife to not have to carry anything at all back up the hill to the house, so that's a win all around. And the more we keep him on his own bike, the easier it will be to transition the t-a-b entirely over to my daughter. The 2 kids are pretty different size/shape so right now it's just too much work to switch bars/seat around between them, and the t-a-b is just his. Soon it will just be hers.