By any chance, do you have an older smaller Intex around, that still is usable?

This is getting pretty involved, but if you can pretreat the fill water in a small pool, and then transfer it to the larger pool, you can keep the iron out of the big pool.

Also, can you disconnect or remove the guts of the SWCG (salt) system? You wouldn't want to use it know, at best. And, at worst, it's adding MORE metals (copper) to the pool! I'm not sure that EVERY Intex SWCG is this way, but the one's I've looked at ALSO add copper.