The Brazos below PK is one of my favorite paddles. I have never done the full 20 miles in the spring, but minus the winds it should be as good of a time as any. Runoff shouldn't effect this area that much, as the dam offer upstream flood control, and the 1st major feeder creek isn't until 10 miles.
My camping advise is to pack light. You will be drag once every couple of miles or so, but never for any long distances. You are off the main river channel if the river is too shallow to paddle. Camp on high ground, and you will see the plastic midden near the good spots. If they let water out, the river can rise in a hurry, especially in certain spots.
My fishing advise is to downsize, as with any clear water. Also don't pass up the first 3 miles, as I think the long riffle to Garland creek offers tremendous fishing. Water with visable movement that is over 1' will hold smallmouth and spotted bass. Largemouth will be in the more slugish water. My last trip was pretty good and we used 2-4" white and/or chartruese minnow baits. You might be just a little late for the trout fishing, but the striper fishing should be good at that time. Large trout patterns should work.
I have a few favorite campsites.
1) 3 miles - directly across from the mouth of Garland Creek. I have paddled down, camped, and paddled back up from here. Fishing is pretty good, even though you are towards the back end of the pool.
2) 6-7 miles - riffle and high gravel bar on river left. This is a small pool of water, but offers a great campsite. The house sized boulder across the river, hides a beaver dam.
3) 9-10 miles - Long island. The end is my favorite campsite. Fishing is lousy, but this is where crawford hallow comes in, and it is a fun place to adventure.
4) 13 miles - If you have read graves book, he talkes about 3 house sized rocks, and this is them. On the opposite side of the river is a nice campspot.
It is one of the most scenic floats of my lifetime, but it is flat flat flat.
I will be planning a may trip, and I am doing the same thing. I am going to the Glover River/Little River in SE Oklahoma or the Nueces.
Have a good trip.