For you the best alternative is Ethereum Clasic.
no. not the best alternative but this coin still gives me a profitable. in fact, if I mine never stick to one coin, sometimes I mine in Ethash algo, XMR as long as it's profitable.
So is it better to buy one another RX 580 for now?
actually, i have 8x RX580 8GB but for now, i suggest you don't buy graphics card unless you don't care about
ROI and have no problem with electricity.