As other have said before, T6-9 are a LOT more mechanics heavy than T1-5.
However the biggest issue is that T6, which keeps you out of T7-8-9, not only has numerous mechanics, that come at you fairly quickly... but they are all based on RNG.
Bulb in the center before a Floral transition = loss of melee dps.
Double Vines and Honeyglazed initial hit = potentially dead dps.
Double Vines and need to be eaten = damage still ticks while being devoured, if the other 2 people dont break the vines, you die.
Double Vines and Floral target = again, possible dead if the people you are tied to are not being proactive.
Basically the fact is that every primary mechanic, Vines, Floral, Bulbs, and Honey, all have an rng component to them. The bulb in the center just happens. But the problem is that the other mechanics can target anyone but the MT (Floral cant target OT), and you can get SEVERAL of them at once. At that point, you have to do your job, but now someone ELSE has to do their job to make sure you survive.
Also worth noting that in the case of Vines, if you have to be eaten or direct Floral Trap, and the person you are tied to goes into a Briar... yeah, that vine isnt going to break, hope you have a regen ticking on you.
This kind of design is what forces people into statics. The core idea behind a static is that you KNOW that everyone in the group knows what to do, or is willing and able to learn what to do, and doesnt suck. If they do suck, and cant learn, you just kick them from the static.
Honestly T7 is easier than T6 simply because there is next to no overlap of mechanics. You get Voice, OR you get Shriek. One person is assigned to deal with Renauds, and that is what they focus on. No overlap, and minimal dependance on others to not mess up.
I get the feeling that the devs wanted T6 to be a "teambuilding" encounter. Where depending on rng, you will have to depend on your team to get through the fight... Sadly this doesnt always work out. And with the extra rng of positioning, honey, and briars covering half the field... even an experienced group can wipe a few times if mechanics dont line up well.