@JimK I also live and ski on a river with my family. Depth is similar (7-15') and we have always used a portable course. Once the spring and early summer flooding pass, we put the course in on Sunday evenings and pull it out on Thursdays evenings. This helps mitigate the risk of damage during the amateur hour shenanigans that take place on the weekends. To expedite the process of installing and removing, we bought an old 20' junk pontoon boat for $600 and stripped it down to just the deck. The portable course is stored on there underneath a tarp when not in use. A little 9.9hp outboard motor is used to maneuver the pontoon around. With the large flat work space, 3 of us have plenty of room to work and stage the PVC arms. We can have the course in or out in about 20min, probably 15min if we really pushed it. It also keeps all the water, scum, muck and other fun stuff that the PVC poles bring up from the bottom, out of our ski boats.
We have one permanent concrete anchor staged upriver of our course location with a 100' section of cable and old buoy attached to it. When putting the course in, we just connect the portable course mainline to the cable and do our thing. This ensures we always are back in the same spot where there is protection from the wind and light weekday boat traffic . The 9.9hp is more than enough to tension the downstream anchor and properly align the course.
Pending on the amount of rain, the current can have an impact on the position of the turn balls a little bit. We just simply adjust our gate timing (early/late) to compensate for how much the balls have drifted downstream. I know its not ideal, but for a public river its the best we can do.
I know this doesn't answer your original question about the mainline anchoring relative to the turn buoys. Just trying to offer another solution from one river skier to another.
Awesome to hear your kids are into it as well. Definitely makes it more fun and provides another set of hands when needed! My 13yr old has been consistent on the mini course this year at 27mph (finally got her interested!) and my 8yr old son ran the mini course on a trick ski for the first time last evening. Grinning ear to ear as he rounded 6 and came through the gates. Good times!