Personally, I agree with basically everything you said. And for the most part, there's not much to look forward to in terms of direct plot. 1 and 2 have way more 'impactful story moments' except for twice in CS3. CS4's story has its moments, but it throws away a lot of what made the previous titles special and is arguably the worst story I've experienced. While the over-arching story and world building is extremely well crafted and very interesting to me, the way it's told throughout the Cold Steel games is like you said - filled with 'coincidences', rough/lazy writing, awful anime tropes and as an extremely bloated main cast. If you can't put up with that, you're not going to like majority 3 and 4.

That being said - the reason I personally play these games now is for the characters. Talking to NPCs each time the story progresses, seeing how they're affected by the events taking place, learning their backstories and relationships - that's what sold Cold Steel to me. Having access to a large group of them after every main plot development (Thor's students, various townsfolk) made the events of the story feel more 'real' and added a ton to the world. Crossbell has this as well, but CS3 and 4 do it way, way better than all the other games. The payoff of talking to everyone is massive, and it's the reason I love this series so much.

While Sky and Crossbell have (mostly) fantastic writing and characters, Cold Steel utilizes its optional content to the fullest. Side quests, optional dialogue and the interactions between both new and returning characters are all incredible. If you don't care about NPCs though, then yeah. I suggest you move on. Give a listen to the soundtracks and call it a day.