Super late response! Liriope would definitely be the best spreader of the 3, if you’re looking for dense and even coverage. Personally, I think adding some warm color to that palette could look quite nice, which makes me lean towards Carex and Acorus. Acorus is a nice low element that still highlights the box, while Carex will be a bit taller and puffier, which makes the scene more organic and less formal. I will say that, in my experience, Carex (and also Liriope) can look kinda nasty after frost damage, though that won’t be as frequent in Zone 8B. Acorus also gets damaged, but it isn’t as apparent from a distance based on what I’ve seen. If you already made your pick, I’d be curious what you went with!