TFOT are pretty much experimental rock/post hardcore/mathcore... I'd just say mathcore, I guess. (Not all mathcore is heavy. Mathcore is all about weird rhythms and time signatures).
And, yeah, I have heard the new Elenora a couple of months ago. Pretty good. It's not that much heavier; there's just more obviously more uncleans and there's breakdowns, too but, IIRC, Rome is where the art is had some djent like sections so it's not like it's a massive transition.
I've been listening to Breakdown of Sanity again recently. Top tier metalcore band. Seriously... their breakdowns are actually pure sex. Like, they're PWD tier at breakdowns.
https://www.youtube.com/watch?v=TpB9gH6YoIE
Well, i'll just leave the link here just in case you wanna check it out but yeah.