I am currently not in Sinbad campaign but I really want to see more campaigns introducing different payrate that is not based on number of written posts.
My best experience was with signature campaigns that allow me to write in my usual style and they don't force me to write 50+ posts, or minimum amount of posts.
I understand what you are saying, but Sinbad already fulfills both those requirements to some degree. It fulfills the first degree completely. There is no minimum amount of posts you have to write weekly, and the upper limit is 25. And "forcing" is too heavy of a term. I don't remember any campaign where you were forced to write 50+ posts. But there were campaigns where you had to write a minimum of 25,30, and maybe even 35 posts to get the weekly pay.
Regarding the other thing you mentioned about style of writing, I am not exactly sure what you mean with that. You either write above a certain minimum quality threshold that the manager will accept or you don't. At least you don't have to worry about that. One part where Sinbad might hold you back is the number of posts that count in local boards for the weekly payout. It's 5 for smaller boards but the number is higher for bigger locals. I am sure these numbers will go up if the campaign starts counting 40 or 50 posts weekly.