I made a wallhanging just like I make my quilts and had no problem with it.
Measuring through the center of the quilt is the method to avoid waves. I never measure the edges of the quilt, causes too much grief.

I was shown to measure from the middle also. Side to side and corner to corner. I also don't do competition/show quilts

Perhaps since you hand quilt one side and machine quilt other sides, there may be a tension problem. Block it before you sew binding on and measure, measure across the quilt in several places to make sure it is square.

I was having a problem with imagining how to do it until my DIL showed me and I use it all the time on potholders and small items now. Love it! Would rather use that than the double fold but guess it isn't double so doesn't wear well on quilts. JMHO.

I sew all of my borders into strips, then attach to my quilts. That way, I don't have to measure but one time in both directions. If you sew each border on separately, then you have to measure both directions after each border is added, leaving more chance for stretching, etc. My preference is mitered borders, so sewing all the borders together first before sewing onto the quilt works great. I just finished a quilt that had seven borders.

Do you starch you fabric before you start cutting? Also I put a sleeve on the top and the bottom of the wall hanging.
Also try not to over work your edges. Hope some of these help.

The ladies that work at my LQS said they use the batting called thermalom for their wallhangings because it's denser and they quilts hang flatter. I bought some and am going to try.

That could be the reason my last quilt had no waves because I needed longer then WOF and did cut on straight grain.

Yes, I just watched several utubes...most show measure middle and sides but one showed only middle...I guess I'll be trying both in my next few projects to see what works best

Tugging a bit is what you need to do. But don't tug the quilt, tug the binding. The measuring down the middle is for adding borders and is very important, but when you get to the binding, the quilt itself should be straight and the correct length.
I lay the binding on the quilt, lay the binding on it and tug just a little to be sure it is taut. Then I put my finger on that spot and sew those four or five inches. If you have WOF bindings, don't stretch too much. If you have length of fabric binding, you can stretch quite a bit. I haven't seen a wavy binding around here for years!