There are projects where I say "I've gotten everything out of this I can" but I still have the need to finish to the point of usability. I feel the pain of being so close yet so far.
I've always found myself able to do my best machine quilting on the borders, especially on my old vintage machine. I got spoiled by having access to a friend's long arm for a number of years but lucky for me, I've always preferred less quilting and a fluffier batting than is currently popular. Just by the nature of your design, as you are finding, so close means the workmanship needs to be more than close enough!
I think part of the problem is the direction of the design and the way you are moving the fabric, plus the walking foot might actually be working against you a bit. In general, unless I am doing straight grids, I prefer a free motion technique to a walking foot. Sometimes I get better results with a regular foot. I like the idea of just going off the edge on the new part.
I might suggest actually using the change in machines as a style thing, start out by trying to mimic the design on both of the completed edges -- then switch to free motion or just get more free and scribble your way through the remainder until you connect the sides.