djb

already thats really good that you've had the spoke tensions checked out by a store, and the fact that you havent had any problems in three tours shows that the setup is working. I personally like putting some weight on the front of the bike to balance things out, the bike handles better and less weight is on the rear wheel, so less chances of spoke problems.
As soon as you start looking into different wheelsets, you'll see that you could find a 36 spoke quick release wheel for under $100 or you could spend many multiples of that on a wheelset or a single wheel. Like everything, there are different qualities of hubs, rims, spokes etc, so there isnt an easy answer to how much a "touring" rear wheel will cost you.

or you can look at the pt of view that you've successfully toured three times with no issues, so dont change things, be careful, continue to use 38mm tires that are not over inflated for your weight and bike load, and they will help with some cushion compared to narrower tires or if you simply have too much pressure than needed.

personally I don't think it is worth spending money on a really cheap new rear wheel, it means that the hub will be mediocre etc, but in the end, it'll be up to you to how much you would want to spend to worry less.
I've toured extensively on 32h 26 inch wheels, but with minimum 38mm tires, up to 2.15 inch tires, and my wheelset has been remarkably tough on some long trips, even dented my rear rim slightly and its still been fine on a bunch of trips afterwards, didnt even break a spoke--but I weigh 50lbs less than you, so this is a big factor, even when I've carried a lot more than you.

won't hurt by asking around at bike stores for wheel suggestions, just take notes of the materials specifics and try to learn what stuff if low end and what is better.