I recently saw a video of a finishing class in which the teacher used a folded paper towel instead of a brush to apply the poly. No brush marks at all in any of the items (mostly picture frames) I've made using the method. The poly is pretty volatile, and it may have thickened from sitting open. Where you live is pretty hot so drying time has got to be pretty short, so thinning the poly a bit probably wouldn't hurt. But do try folding paper towels into a brush shape and applying each coat of poly fairly quickly.
The more I do, the less I accomplish.