I have never had wavy borders. I fold my quilts in half, lay the border piece on the fold and gently run my hand down to the other side, cut border there, lay it on top of other border piece and cut it to that size. I pin the top edge in half and each side of the pin in half, do the same with the quilt and pin both together and when sewing together I hold at the pins and if long, I hold it out and hold between the pins and make sure the pieces stay the same size. Then I do the same for the sides. I don't like square corners on my quilts so I don't miter, I round the corners using a plate to round them off. Could be you could just redo the miters and take up the fullness that way.
Don't stop trying, just keep making quilts and you will eventually get them right. Get a book that shows how to do borders and try that.
Your quilt is beautiful and I can see all the work you have put into it.