I'm sadly forced to write this on phone so you'll have to excuse the formatting~
Darm is absolutely broken and should be quickbanned. I'd rather not rehash too much; it has 2 very powerful sets, both are extremely strong for their own reasons, yada yada. The question of whether it's broken or not isn't really interesting to me; it's brokenness is apparent to almost every high level player. I think suspect vs QB is a better question and while I do think a suspect has merits, ultimately, a quickban will be better for the tier.
In the case of a suspect: we waste ~2 weeks of meta development for a suspect that is rather certainly going to result in a ban. This especially affects the tier in the first few weeks of SPL, and I would argue those weeks are some of the most crucial ones for early tier developments. Should a suspect happen, it would have to be right after Christmas(at the earliest), leaving at least 2 SPL weeks with Darm in them. This will, imo, slow the tier down much more than it needs to.
The upside of a suspect is more formal discussion, maybe a better talk of a GT vs Darm ban(i think a GT ban is more preferable, for the record; Zen Mode is a beneficial keep to the tier and I would compare it to PC Zygarde, but I won't go into it more in this post), and a clearer image of where the community stands.
I honestly don't think that is enough to justify stunted tier growth during the biggest tournament in Smogon. Judging by this thread and interactions with community members, the result will, in my eyes, almost certainly be a ban. A quickban avoids all of these problems, and let's the tier evolve more naturally. Quickban it.