This isn't a solution I would have come up with, but I inherited a setup with pentair big blue housings and epm-20BB carbon block filter cartridges, and they actually work pretty well at polishing. At about 50 bucks each, they aren't too crazily expensive for how much volume of product you can treat with each one:
https://www.homedepot.com/p/Pentek-EPM-20BB-20-in-x-4-5-8-in-Carbon-Block-Water-Filter-PENTEK-EPM-20BB/206017179