Flourite and Eco have great CECs and are great low maintenance substrates (unlike most soil substrates, and sand is obnoxious IMO), especially for a low tech setting where no additional ferts may be needed anyways. Not sure what you mean by "only trace Fe" since Flourite has a huge iron content and probably more than plants would use up in a very, very long time (again assuming low light).
I've never used SMS or Turface but lots of people have trouble keeping plants rooted at first due to how lightweight it is. It's also gotten a bit harder to locate of late, especially now it's out of season. It'd probably cost as much to buy a 50lb bag of Tuface/SMS (and 35lbs would go unused) as it would to buy one bag of any of the others.
Onyx sand is another good option.
CEC stands for "cation exchange capacity" and referrs to the ability of substrates to pull nutrients from the water column and hold them in a form that can be used by plant roots. Fired clay substrates (such as Flourite, Turface, etc) tend to have high CECs, unlike completely inert substrates such as sand, quartz, gravel, etc.