I just noticed this thread, but it sounds like you've got it figured out.
Bottle feeding bunnies does not work well, so I'm very glad you've got a doe willing to nurse them!
You can probably wean the one set of bunnies when they are 4-5 weeks old, and then hopefully you could have the younger ones live with the foster mother from that point on. That would reduce how many weeks you spend carrying rabbits back and forth
Edit to add: or would it work to assign each litter to a cage, and transfer the doe every morning and evening for the next week or two? So she'd spend half her time with each litter, until the older ones can be weaned and then she'd just be raising the younger ones.