I´m kinda burnt out so I´m not listing a detailed overview of the E4 and N/Ghetsis, if you don't mind. Instead, I'll just give my final verdict on the pokemon I've used to finish my playthrough. I still have save states, so if anyone really wants detailed matchups for the E4, I can provide them later on when I have some time.
Simisear -> B tier, makes a case for A (like Panpour)
Pros: great availability and early evolution, good offensive stats, amazing coverage, few bad matchups, surprisingly useful support options in yawn and wisp, does NOT fall off thanks to the power of fire blast
Cons: bad early game, slight 4-slot movepool syndrome, not very bulky
Apart from the disappointing early game, Simisear has very few flaws. It is a very reliable offensive pokemon with amazing matchups overall. It even has a good niche in yawn, which helps it set up work up. It stays relevant in the late game because of the sheer power of STAB fire blast. I found Simisear to be extremely solid and reliable overall, so I would put it in A. I tested it twice now, and it hasn't disappointed me
Seismitoad -> C/B tier
Pros: good availability, amazing typing, decent coverage, good matchups
Cons: very average offensive stats, no earthquake, falls off in the late game
This one is quite hard to place for me. It was pretty good during the mid game, pretty good matchups and such, but it just felt weak after like Skyla. It often misses important KO's. I'm torn between C and B.
Archeops -> A tier
Pros: insane power and speed, great coverage, arguably the best route sweeper in the game
Cons: defeatist obviously, requires TWO backtracks to use effectively (one for Archen itself, and one for rock slide), disappointing major matchup spread
I don't think this is S worthy at all. Don't get me wrong, Archeops is great, but I think it has too many flaws to be in S. It's matchups are pretty mediocre on the whole, especially the E4 (I played around a bit, nothing too detailed). It also requires two pretty annoying backtracks. It breezes through any none-major matchup though, so I think A is still justified.
Ferrothorn -> B tier
Pros: great typing, near indestructable, curse + gyro ball is amazing, great matchups, iron barbs often secures kills
Cons: beyond slow (vulnerable to hax), some pp problems
Ferrothorn is really good. If it came a little earlier, I would put it in A, even. It's main selling point is the curse + gyro ball combo, which is just insanely powerful. It can tank its way through most major matchups with ease. The only problem other than it's low speed is that gyro ball only has 5 pp, so you probably want to keep metal claw. But since you also need a slot for curse and power whip, you won't have a slot left for coverage. Then again, you don't really need coverage most of the time since gyro ball does major damage even to pokemon who resist steel.
Stunfisk -> D tier
Pros: great bulk, interesting typing and coverage moves, great status spreader, few truly bad matchups
Cons: comes late, slow, mediocre power, terrible route sweeper (needs lots of healing support), doesn't get some moves it should have gotten (earth power, volt switch and light screen come to mind)
It wasn't a total liability like I thought it would be. It actually has some decent matchups. The problem is that it has power issues, and also speed issues. It's bulk is good, but not nearly on the same level as Ferrothorn, so the speed issues are a lot more relevent too. I just wish it came a little earlier because It would have been amazing against Elesa and Skyla.