I don't know what's going to be the actual BiS, but the stat system is simple enough that melding for common sense is going to be good enough: Critical unless you get more value from Determination (it always depends on the caps), Accuracy if there is an uncomfortable jump between sets you could be equipping. In the end, we can only accept that we're going to have this much skill speed at i240, and even if you can pentameld Eikon gear and even if that is theoretically better, I try to cater to players of average-size pockets. Like, you will eventually get your hands on the grade V materia if you play until 3.4 and you really want it; there's Void Ark weekly for now, you can spiritbond Sky Pirate gear in VA/dungeons, Diadem spoils may be an option for you, and probably more sources coming later. IV should be the budget option, quite reasonable while you grab Vs.
I don't talk about too much about speed caps because I already do enough theorycrafting in a vacuum as it is. We live in a world where people make mistakes, Fey Wind / Arrow is helpful, changing gearsets for accuracy between bosses isn't practical if it involves remelding, bosses jump away, latency is a thing, BiS is often slightly obsolete within 3 months and definitely within 6 months, etc.
And yeah we don't really know the accuracy caps for Midas yet. I've considered trying to find some simpler pattern behind it, but no dice so far.