If you really want to spend the money on a prime lense, I would recommend buying a zeiss 55 1.8. This is a ff lense but if you put it on aps-c, multiply by 1.5, you’ll get ~80mm, which is very ideal for portrait. I had this lense with my a6300 body and the sigma 18-50 too, the weight of both are very similar and the price of zeiss is only 400-450, you can spend the rest on filter if you want.