I think the first thing you should look at is tension. Since you needed to go to a size 18 needle to prevent thread shredding on previous quilts, this indicates to me that your tension may be too tight. I highly recommend following Jamie Wallen's method for setting tension as shown in this video:
https://www.youtube.com/watch?v=Q1mRhcquZTM
Here is a Nolting webpage that explains different causes for skipped stitches:
https://www.youtube.com/watch?v=Q1mRhcquZTM
How tight is your quilt on the frame? Many beginning quilters make the mistake of having the quilt too tight. A rule-of-thumb is that you should be able to grab an index finger poking up from underneath the quilt. This allows for about 4" of "give" in the quilt.
Batiks are notoriously trickier to quilt on a frame because they are so tightly woven. I do not think you should downsize the needle. If changing the tension settings does not solve the problem, you may need to think about adjusting the needle/hook assembly gap or timing.