Hello, before I go on, I'd like to congratulate you on your success, and I hope you get a suspect vote, since you indeed look like a knowledgeable person.
You mentioned that you never really got to see Garchomp at it's height. One of the sets that really pushed it over the edge 4th gen was the SD Yache set. That trolly Speed stat you mentioned also happens to work on Randorusu/Landlos, and with Yache berry taking away your clean OHKO, you'd be hard-pressed to stop it without a Rock Polish. A once-Danced Outrage looks pretty capable of cleaning you up if Scizor and Garchomp are gone. Your own Garchomp can revenge other SD versions of itself, but watch out for Sand Veil. Speaking of Garchomp, if you are running a Scarf, Fire Blast is almost always the superior option, as it does more damage to Skarmory and Nattorei, even with a hindering nature, at the cost of some accuracy. Scizor's Bullet Punch will still 2HKO it if you can get it locked into Outrage.
If you want something to replace Stealth Rock, I personally would go with Stone Edge, and if not that then I might give Crunch/Payback a try. The problem with the many MixTars I see is that they lack a strong Physical STAB, since Pursuit will rarely OHKO the target if it doesn't switch, such as Specs Latios, whom I highly doubt will be bothered by its other moves. Stone Edge is a stronger STAB option, and Crunch is a more accurate one. Payback I threw in because Tyranitar is, of course, slow, and will be a 100 BP STAB move with 100% accuracy a large percentage of the time. Burungeru and Rankurusu will laugh at that, but T-Tar doesn't like their Will-o-Wisp or Focus Blast, respectively, so you probably won't keep it in on them.
That's all I have to say that isn't a horrible idea in the making. To be honest, I think you have done this up to near perfection, as is evident by your success, but the fact that rain teams are so common makes me wonder how you beat that Water + Ice weakness. I hope you can find some use out of my advice.