A 30 foot span sounds like fun, nice long sways side to side!
It sounds like the more weight you put in, the lower you get, so stretch is definitely coming into play. Also, it sounds like it might be stretching more over time (hours). Before making changes, I'd figure out exactly what is stretching.
You'll need a helper for this. Get in and have your helper measure each side as precisely as possible. Add more weight and remeasure. Repeat after being loaded (the hammock, not you) for a few hours or overnight. See what changes.
In a friend's setup, his tree straps were nylon and the source of significant stretching. I would recommend wrapping the webbing as many times as can reach, and then attaching to all the loops at once, thereby minimizing stretch there. You can't do that with a marlin spike. Since this isn't backpacking, I'd just use a big cheap hardware store steel S hook.
Remember that everything stretches, even amsteel, and the amount is approximately proportional to load and inversely proportional to strength, so changing from 2000 lb strength to 4000 lb will cut your stretch in half. At 30 feet, even just a 1% stretch is almost 4 inches.