Though I'm strictly a flat bar single speed or Rohloff kind of guy, I like the Shimano Soras - they've been around a long time. That said, they are entry level. I would take a very careful look at the the shifters to make sure they haven't worn in a way to increase drag or have picked up some rough edges, and that surfaces - especially the outer cable guide and joint - contacted by the cable are smooth. (I'm always reminded that the less expensive the parts, the better mechanic one has to be.)
Though the original cables on the 520 should have been stainless steel, if one's not careful, replacement cables can be prone to corrosion and failure at the points of contact. Sometimes, just moving to a more expensive system like Jagwire's pre-stretched, slick, polished stainless steel cables can result in longer life (and often, surprisingly smoother shifting). (I use Jagwire for all my braking needs and have never seen the appeal of hydraulics - especially on long tours.)
On the other hand, if you want something nice to replace the Soras (and you probably do), you could try the familiar 6510s, but you're going to get used equipment and older engineering. Cruising Ebay might give you some ideas.
Personally, I would take a hard look at the Shimano-compatible Microshift R9s... This is a link to their drop bar shifters, but they also have bar end shifters and other options. https://www.microshift.com/models/sb-r493/ I see there's a few pairs of the R9s on sale at Jenson.
Make sure you have fun.
