You are very attached to this idea that it will be like other legends, but can you imagine, and like I said, nakaba changes roles
The one who founded the Round Table would be meliodas, and instead of characters like kay it would be characters like donny or chion (lol) , Meliodas would play the role that the good King Arthur would play for the 4kota, except that at some point Arthur's heir constantine should appear to be the leader of the round table in some day, Another thing, nakaba seems to be based on the Welsh round table, there is no kay or bedivere because they are knights above it
The series is obviously not an adaptation of the legends so Nakaba is free to put his own spin on things, but he does tend to remain faithful to the central concepts like Arthur being the king of Camelot and the wielder of Excalibur and so on. I don't see Nakaba making the Round Table into a group led by Meliodas and I also doubt the likes of Donny and Chion are going to join the Knights of the Round Table given how Nakaba handles his characters' names. Naming the KotA after some of the most famous KotRT was a very deliberate choice and he even went as far as to pair them up with their canonical love interests. The Chaos Knights were the prime candidates to be the KotRT given that they serve the role of Arthur's knights, so the fact they turned out to be a different group suggests that Arthur is eventually going to have a different order of knights serving under him. This is somewhat similar to how Arthur initially wielded Sequence instead of Excalibur when he first appeared in NnT. That was not because Nakaba had decided to replace Arthur's most famous sword so that Meliodas could be its wielder, he just wanted to save it for later.
It's not Arthur that's the problem, it's the protagonists. Arthur has the power of one of the strongest beings, but he and his force's are still outclassed by the Sins to the point that he's only remained around this long because he's hiding in another dimension where they can't get him. True he's apparently weaken (though honestly it seems liek he's just not using some of his abilities when he should) and we know that Arthur will gain something/someone who will allow him to eventually defeat them, but alone the Sins are superior (since they already defeated the other two gods) and the KoA seem like they will be even stronger. If Arthur with the power of a god is below them, what power could be granted to someone to make them a greater threat that would make sense? That's not saying it couldn't happen, but it would be terrible storytelling to just have some even stronger villain just appear...
No, it's definitely Arthur that is the issue here. Remember that it took all seven of the Sins to defeat the DK, and that was with Escanor burning out his soul and Merlin pushing the group beyond their limits with her spell. Likewise Cath had no qualms about fighting Arthur and six of the Sins at once. The Sins are powerful but not so powerful that nothing can stand up to them, especially now that they're down to a mere five members. Arthur should be much stronger than the DK and Cath combined so why is he so afraid of facing a weaker iteration of the Sins than what the two of them did? Not only that but he also got humiliated and even intimidated by Lancelot, who went on to have a tougher time dealing with the below Archangel level Nanashi. Whatever happened to make Arthur turn evil has seemingly also cost him the majority of his power, which is why there are no problems with a threat greater than him showing up at some point. I brought up Cath earlier and he's an excellent example of how easy that is to accomplish. The Sins were unable to deal any damage to him while he managed to easily trap them in an illusion without them even noticing when it happened. If not for Arthur saving their asses the Sins would have all been killed. This current evil version of Arthur is a complete clown in comparison to Cath.
Ominous Nebula has no hax. It's just Zeldris controlling his PoD and slashing rapidly. Full Counter reflects any magic attack regardless of its power, Snatch and Invasion's effects can be reduced depending on the person's physical and mental strength, but they still work. Certain Invasion techniques like Search Light always work, no one can stop Gowther from reading their memories. Perfect Cube isn't unbreakable but even AM Meliodas VS The One Escanor's fight wasn't able to break it.
The default is that spatial attacks like this can't be tanked. When Merlin used Teleportation, you can't resist it via strength, you have to dodge(which is what Galan did), Gawain teleported Rhitta in Galan's gut and he didn't resist.
For Berto to do that he would have to target the DK at a vital spot and with a slash large enough. That's extremely unlikely to happen. If we create all these specific scenarios it's pretty easy to change the outcome of many battles.I said that Percival and Tristan are better equipped because they can heal if the attack lands, but it's not like Lancelot and Gawain don't have anything to deal with this. Lancelot can absorb magic, Gawain has Absolute Cancel and teleportation is spatial manipulation as well.
I didn't say Ominous Nebula has hax, I was pointing out how Escanor powering up caused the magic to stop affecting him entirely. I brought up his hair because that would still be pulled towards Zeldris even if you were to argue that Escanor's physical strength allowed him to resist the force affecting the rest of his body. We see this during that same fight when Chandler and Cusack's hair is affected even when their bodies are otherwise stationary due to them holding onto a wall. Escanor essentially started completely ignoring the effects of Ominous Nebula, which is in line with how Gowther's magic was reduced to 1/10 of its normal power against the SD. Essentially it seems like a sufficient level of power makes it possible to largely negate magic abilities.
On that note, I'm not convinced that spatial abilities are an exception to this rule. You bring up Merlin's Teleportation as an example but have actually already seen that magic be disrupted. Back when Merlin and co. were heading to fight the elite demons in Camelot she mentioned that the "high density dark zone birthed from Meliodas (and the Commandments)" made it impossible to teleport into or out of the castle because of the shell of darkness around it. The phrasing is important because it makes it clear this was not the work of some anti-teleportation spell cast by Chandler, but rather just the effect of raw magical power emanating from Meliodas and the Commandments he was in the process of absorbing. If you believe that teleportation is in the same category of spatial abilities as Beltreipe's cuts then wouldn't Beltreipe be similarly unable to cut through that kind of a dense zone of darkness? That shell was eventually shattered by the Original Demon releasing its strength, which suggests that anyone on that level or above could likely exert their raw power in a similar way as Meliodas to disrupt teleportation and similar spatial abilities. This is in line with the magic resistance idea I brought up in the previous paragraph.
