I am not an expert on this topic, by any means. From what I have gleaned on the internet, it is important to launder a batik backing before layering. Many quilters have reported more difficultieswhen quilting batiks (especially batik backings) because of the tighter weave associated with batik fabrics.
Suspending your quilt may help. Here is a link to a commercial suspension system, although you can also find directions for making one yourself with PVC:
http://www.jennoop.com/quilt-suspenders.html
This thread on inconsistent tension and fixes may be helpful:
https://www.handiquilter.com/forums/...istent-tension
Are you winding your own bobbins? If so, you might want to switch to magni-glide pre-wounds.
You might want to consider replacing the check spring on your upper tension. They can wear out.
Instead of winding the thread twice around the tension discs, you might want to try adding additional thread guides on the machine -- such as taping short lengths of straw before and after the thread guide to try to keep it from jumping out of the discs, or possibly taping a closed safety pin (could be bent to stick out a bit from the machine) and threading through the circle portion of the safety pin.
What brand and type of needle are you using, in addition to size?
There are so many possibilities, I would start with the easiest things to try and work from there.
Edit: I honestly think there is something other than tension that is causing the problems. I would leave tension as-is while experimenting with some of the other things, and I would be careful to change only one thing at a time. The fact that your practice piece can be fine but you still have problems with the quilt itself indicates to me that the size and weight of the quilt may be the primary issue -- something a suspension system would resolve.
Switching to the magnaglide prewounds would be an easier place to start, though.