Update: That's what I'm going with - running the >750ppm well water first through a conventional NaCl softener (after which the TDS is still about the same, which makes sense, as it's just swapping Na ions for those coming out of the ground), then running it through the consumer-grade RO unit (membrane + 3 filter stages) to strip out the Na, after which the TDS, according to my cheap little EC meter, is close enough to zero. This little under-sink-style unit, really just built for generating drinking water, is supposed to be good for 50 gallons/day, so I think it's going to be fine for the forseeable future. I've had it for years - installed it when we moved to this house, but mothballed it because it wasn't producing any volume. At the time I didn't bother to figure out why, but now it's clear that it was because the supply pressure wasn't high enough. I've jacked up my pressure tank and jet pump, and things seem to be working well now.