Some of your electric checks mentioned cannot beat thundy-t if it gets a nasty plot up. +4 252 SpA Thundurus-Therian Gigavolt Havoc (175 BP) vs. 248 HP / 144 SpD Venusaur-Mega: 351-413 (96.6 - 113.7%) -- 81.3% chance to OHKO. WIthou stealth rocks, this calc shows that if thundy-t gets two nasty plots up (1 as the mega venusaur tries to switch in or after you kill a pokemon and have a nasty plot up). 0 SpA Venusaur-Mega Sludge Bomb vs. 0 HP / 4 SpD Thundurus-Therian: 138-163 (46.1 - 54.5%) -- 55.9% chance to 2HKO. Sludge bomb is not near a OHKO. SImilarly, Thundy-T also breaks past ferrothorn. +4 252 SpA Thundurus-Therian Gigavolt Havoc (175 BP) vs. 252 HP / 168 SpD Ferrothorn: 353-416 (100.2 - 118.1%) -- guaranteed OHKO. 0 Atk Ferrothorn Gyro Ball (150 BP) vs. 0 HP / 0 Def Thundurus-Therian: 102-121 (34.1 - 40.4%) -- guaranteed 3HKO. Marowak can be pressured by stealth rocks, and it does happen to be a pretty good check. Excadrill is also a decent check, but that is only two, and when you told the list goes on, could you expand a bit. The point of thundy-t is primarily to completely destroy stall.Ive been using Thundurus Therian and ive found it fairly underwhelming. The tier is littered with diverse and splashable electric resists. Marowak, ferrothorn, excadrill, venusaur, the list goes on. This forces Thundy to rely on a ton of team support to lure in and KO these problems, or to run 3 attacks which makes it kind of a lame cleaner compared to the likes of salamence, volcarona, magearna, pheromosa, landorus, etc who boost power and speed in 1 set. I wouldnt put it any higher than B.
To expand a bit on the argument, it is immune to arena trapping, and is arguably the best pokemon vs standard stall, basically killing either chansey or clef. Other then that, bulky offenses have a lot to fear from this monster, and so do offenses if it manages to get an agilty. I would support a potential rise to B-/B. It is better than other C rank pokemon, there is much better than other C rank pokemon, like it is almost always better than xurxitree, and is better than Garchomp-Mega, empoleon, jellicent etc. as it has a much larger niche. Comparing it to C+ rank pokemon like minior, alomomola, necrozma, whimsicott etc. It again fullfills a much larger niche, and does many things like a cleaner that helps a ton vs stall.