Bunching after attaching the edges says to me that the angles on the edges were slightly smaller than the spaces they were filling. I try unstitching the last pieces, and taking the seam allowance on a *very slight* angle. In fact, you could unstitch just one side of the last piece, lay the whole block flat (assuming it's still flat at this point. Then you can actually mark where the seam allowance on each piece should be (ie, where the 2 sections overlap) and restitch.
Are you talking at the straight borders causing the bunching? That would surprise me and I don't know how that could happen. But I'd do the same thing. Unstitch, lay it out, and adjust seam allowances to make it fit better.
They are gorgeous blocks. I hope you figure it out.
Oh, by the way - restitching the center is a theoretical option too - just carefull unpick the very center of the star and restitch those areas to adjust for the bulk.