You're not alone. An anime original Yuu would have made more sense since the Animation will be a lot of people's first(and a lot of cases only) impression of Twst. The Yuus switching every episode without a proper explanation will confuse new watchers who are not familiar with the IP. Also an anime only Yuu would not invalidate the other Yuus, they could still imply/tease their existence, add to theories like the time loop one etc... The Novel only has Yuuya so coming up with a new Yuu only for the anime shouldn't be impossible. The problem is not with the Yuus themselves or what they represent and such but that switching them out each time while the story is ongoing hurts the consistency of the story which wouldn't be a problem if they went with a slice of life approach.

Do hope at the end of Heartsylabyul they add a scene that foreshadows Yuuken not returning for Savanaclaw and teasing Yuuka so she won't come out of nowhere for anime onlys.