Another thing to think about is if you'll go full frame any time soon. The 35mm will work on a ff, but the 18-35mm won't. I've often thought of giving up the endless quest for more gear and selling everything to get a 7Dii with that 18-35mm and Sigma's 50-100mm, and call it a day, but I've been bitten hard by the desire to keep getting too much, too nice (for me) gear.
What they're all saying about longer focal lengths for portraits is something that I never wanted to believe until I experienced it. I gravitated toward wider lenses because they'd hit focus a little more easily and I'd lose less shots due to camera shake, but after goofing around and taking pictures of my wife with a 600mm lens at like, F8 (and still seeing beautiful creamy bokeh seemingly a million miles behind her crisp silhouette) I was sold on the longer focal lengths for portraits. The look is night and day after you really put in some effort with both. I sold my Sigma 35 to buy a Rokinon 85mm F1.2 and have had zero regrets. Sold my 16-35mm F2.8L ii to get a 70-200 (buying any day now, debating between F2.8 and F4). I also picked up a 100mm F2.8L macro lens which is crazy sharp for portraits. These days I almost want to ditch my 24-70 because I've grown to prefer the distance between myself and my subject.
Give your 50mm another chance, and really try to find something to love with it. 85mm to 135mm takes pictures of people, 35mm to 50mm takes pictures of people in specific places. It adds context. I take a 50 on vacation so we can see the beach behind my wife, I take a 100mm for portraits, when the background isn't important.
Sorry for the long winded reply, this topic just hits close to home with what I've been learning in my own experiences recently.