I use my Sac Plat everyday to tote files, documents, binders, office supplies and then carry one of my favorite LV handdbags along with it.
I love my Piano Cabas for work (and I also use my Gucci Abbey) but since you already have the BH AND BV, I would suggest getting a non-monogram. Why not the Epi Passy GM? Or something in the suhali line.
Either the Batignolles Horizontal or the Cabas Mezzo.