Well, I'm wrapping up this phase of the experiment. I did a full water test (except I forgot CCs... but that's ok for now). I'll test CYA when the water sample warms up (and the sun comes out again).
FC: 5.0
pH: 8.0
TA: 120
CH: 200
Salt: 4000
SUCCESS!
Clearly, the CH has gone down and the Salt has gone up. Salt is a little high, but I found last year that my SWG reports a slightly lower salt level than my drop test, and it should be good up to 4500ppm.
I added plumbing to my set up that lets me connect a hose through a y-adapter so that I can select the pump (and pool water) or my spigot as input to the softener (and output to the pool).
So, for anyone else considering this.... it worked... for me. I'm not sure what the ROI is exactly. I had to buy the softener, plus a pump. I had to buy casters and build a platform for the set up. I had to buy several bags of salt. And I don't know the longevity of the resin, and I'm not exactly sure what I am going to do come winter time, but that's a future me problem. But, it really did work, and hopefully I can keep using it for top offs.
I'm hoping to reassemble the pool equipment and open the pool next weekend.