Some of the numbers don't look right.
645 lb tongue weight seems too low for 6500 lb loaded trailer. Tongue weight under ten percent, could create sway issues.
Average tongue weight being twelve percent, would equal about 800 lbs tongue weight.
Weight of WD hitch (about 100 lbs) must be added to cargo weight. Add that weight and the available payload is less than the tongue weight. Once the combo is all hooked up and run across the scales, I think payload and GVWR will be exceeded. If you're not over, you'll be so close, you will worry about anything or anybody loading up unplanned for, as it / they will put you over. I've been there, it kept mother in law from riding along, and she didn't like getting on the scale.
I can tell you from experience, towing that close to max (and possibly a bit over) weight ratings, can be very unpleasant. Prior to traveling, you'll need to watch weather reports, morning, noon, and night. If the reports call for any kind of head winds or cross wind, plan on back aches, neck aches, white knuckles, and cramps in arms and legs. The pains will subside in 36 - 48 hours. Just in time to head home from a weekend camp out. Hopefully you will have the flexibility to travel, only when the winds are with you.
I think you should look at lighter trailers.