The thing about Yuuki is that he's not directly opposed to Rimuru. He wants chaos where Rimuru wants stability so they wind up bumping heads because of it. It just so happens that Rimuru screwed up a lot of his plans (he landed right in the middle of the Clayman situation while it was already happening and wound up stopping it for instance), and Yuuki retaliated in response. I really like their dynamic, it REALLY reminds me of Medaka Box (especially since Yuuki and Kumagawa are practically the same character, both wanting chaos and not usually being directly antagonistic to the MC), though it's not shown well in the anime.

Minor detail so easy to miss if you weren't paying attention, but demon lord Deeno (sleepy guy) from Walpurgis was introduced as a fallen angel.

Regarding the Seven Days, keep in mind Hinata defeated them which was stated in ep 2 where she also copied her powers so the Seven Days were shown weaker than anticipated. Also Granville the old guy despite being a hero decided to work under Lumi which we don’t know why yet.

The Seven Luminaries (Seven Days Clergy here), are some of the strongest Humans in history, it's because of that, that they were so cocky towards a newly born Demon Lord (since they've likely seen a few Demon Lords born in their time) and why they were taken off guard when their attacks didn't work (Disintegration is one of the strongest spells in the series it likely could have severely damaged Rimuru if he didn't block it, it also shows how ridiculously powerful and dangerous Diablo is since he just took it with no protection). Granbell (Granville here) is the old man we saw talking in the beginning of the episode who was a previous Hero of Light (anything surrounding him is spoilers so I'll leave it there), we saw his soul leaving the Luminaries body and going back to him last episode showing that he was puppetting one. It's not really your fault you feel this way about the people being shown, the anime has been going out of it's way to downplay every single threat that's been shown so far since season 1. Rimuru has actually had a few fights for his life but they're shown so flippantly in the anime that it looks like he never struggles. The directing is absolutely terrible. Anyway the enemies he's been fighting here (the Battlesages that Diablo fought, the Seven Luminaries, and obviously Hinata) are some of the strongest Humans in the Western nations despite how they've been shown in the anime it's why they're being given so much focus.

7 days priests and cerberus [ the eastern merchant guild's name , one of the heads is that Damrada guy ] have deeper ties then you realize , we saw Granbell Rosso escape to his stronger main body after his seven days body got wrecked by Nicholas { the priest guy who wants to be Hinata's beta house husband }

Yes he said Diablo isn't unbeatable but he pointed out if you're ready to lose the whole crusade of men to beat Diablo

tldr, Granville is someone who past his prime, long time ago he was someone who's capable to be on par with Luminous, but then got weaker "somehow"

Why you guys just don’t get, in 龍傲天 type of series, no villain will be really a threat and that is fine.