I recently finished quilting my king size quilt on my dsm (and a vintage machine at that - a Singer 15-91). And I was worried about the same things you are worried about. So - decided to cut my batting into thirds. I chose to do meandering on the entire quilt, and I started with the middle section. I used a fusible tape to attach the batting back together. I used 505 spray for basting.
When reading about cutting the batting for a larger quilt, it was recommended to cut in in a wavy pattern, not a straight line. I ended up with an extreme wave, which I would not recommend. Next time (yes, I would do this again) - I will do a gentle wave. Also, I think it would make it easier to add back in the batting pieces if the stitching stopped in roughly a straight line down the quilt a couple of inches in from the edge of the batting. The quilt was so big, I kind of got lost as I meandered around, feeling for the edge of the batting. Some of my stitching came perilously close to the edges.
Honestly, I think I could have done this without cutting the batting in thirds. Yes, it would have been tight in spots, but doable. Next time I would probably just cut the batting in half. I have to admit it was nice to be able to move around the middle of the quilt easily. I puddle my quilt when I FMQ.
I am attaching a picture of my quilt while I was working on it, as well as the end result (almost end result - I still need to do the binding). I also added a picture of my favorite sewing machine - the one I used to FMQ on this top.
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