Looking at their chart, the FC024 looks like it'd be best for what you're after as it'd most closely mimic the treatment provided by a regular water softener.
Disclaimer: BUT I'M NO EXPERT so my advice is worth what you paid for it
I'm looking at it this way- sediment filters catch "big stuff", but the softener oughta help deal with the mineral content that makes you water "very hard".
I wouldn't expect much relief from upping the ONR concentration as you'll still be getting the "mineral film from the hard water". Replacing that with the much lighter film that "soft water" leaves behind might be the next best thing to using RO/DI water.
[Repeat disclaimer...]