The amount of flex you go for is really a preference thing. I love my flex 1 Fattail but it does feel a little iffy closer to the top end. That being said flex is going to be an issue especially with such a big battery. You will definitely run into issues trying to use a flexy deck with an enclosure that stretches across the majority of the underside. If you look at my Fattail build you can see how far back the enclosure is mounted, even then I had to move the bolt holes back from the corners so I wouldn’t have to worry about it.
You’ll have to go for split enclosures if you want too keep everything working. I forget who but someone had a Icarus with split enclosures that seemed to have a good amount of space in them. You should be able to find it if you search the build section.
Thank you very much. Next thing Ill do is check out your build then the Icarus build.
And yeah, splitting the batteries and having a 6s pack at front and back respectively seems like the only option.