If you're still holding on to 520, better to hold on to 8.1 as well. Most 520 users find W10M to be 'okay', and not as smooth as WP8 or 8.1. This is due to the hardware limitation of your device.
I'd suggest you to upgrade to either the 640 or the XL version of it. Kinda cheap nowadays, and definitely a huge upgrade from your current 520.
I'm now using Lumia 830, but I came from a 620, so I kind of know how hard it is to part with a phone that functions well according to your needs. But the thing about electronic device is that, they don't have a long life-span. Sooner or later, you'll have to replace them with a newer, up-to-date device.