I have JF moderately emblemed and all of your maxed reds as well, and while I do like him, I do think a lot about how poorly he works with the other red heroes I use. That being said, I specifically chose him over Khagan because I can significantly load him up with emblems that Khagan will never see, as my ranger roster is so much deeper than my wizard pool.
Regardless of your emblem situation, I doubt Khagan is a better choice. I think If you are bringing a 3 red stack you probably want everybody to do some damage, and replacing Grazul with JF will bring a lot more punch. His specific fit with Marjana and Mitsuko is somewhat clunky, but the do all still work, and the crit bonus is nice. The other clear case for JF is for your green titan team, since your reds all have pretty poor base tile damage. JF has the highest base attack to begin with, and I value his crit bonus much more than Khagan’s defense and mana buffs.
Ultimately Khagan is a slow hero who isn’t impactful enough to make up for his mana speed. If you are sitting on a horde of ranger emblems and you will make a super Khagan it would help his case, and if you are just too bothered by the poor fit between JF and Mitsuko/Marjana that’s fine too, it won’t break your roster to max Khagan first. Personally, however, I had no qualms skipping over Khagan for yet another round of rings.