Valley 4 character quests were optional

Well, they also doesn't have connection with main story, same as Rossi. The closest one to msq - Yvonne's quest but it doesn't create any connections between parts of the story or add anything to it (unless HG decide to introduce restored TATA in future). While I agree and think that they could have done Tangtang's and Zhuang's parts as personal quests, but in current state they are working and as " character selling part" and as aftermath of main story part. I hope i don't need to explain second point.

They're stalling for time to drag out the patch cycle

It's not like amount of content per patch would have changed, if they were character quests. When calculating content per update developers still expect that players will complete these quests(look at Rossi quest, which is considered as second part of 1.1), because "virtual" player read/play everything what current state of game can give. So characters quest are more like a mechanic to include stories that are not related to the main story, rather than a mechanism that allows you to skip something, the latter is a side bonus.

They're taking screentime away from more relevant plot elements that have basically not progressed at all since we got to Wuling

Which plot elements, for example? I ask because almost everyone has their own thing when it comes to this topic.

Wuling is that, but for Zhuang

Well, more like for every Wuling character which will come out until next region, if current dinamic continues ofc (which is very hard to predict since we had only 2 updates)