I don't think you'll see a ton of people. Ill tell you that you wont be able to use the preferred shuttle route (Bear Camp) You'll have to go the long way around, which will be a pain but its a cool drive. You'll follow the coastal highway home.
I agree most rapids wash out, I don't know if you'll see 20k, its possible, don't forget its still dam controlled and they tend to hold water back, a big rain and or snow melt event could get you to that level though.
At 20k You can run right over the top of Rainy falls, SOME of the smaller rapids have large waves but its mostly just squaring up style boating.
IF it its at 20k you could do all 35 miles in one day. Might consider starting higher up or just knowing your going to camp more and raft less.
I'm planning an April trip if we get a nice weekend as well, great time of year to be down there IMO