Thanks to all for your responses which are definitely helping me to clarify my thoughts.
I was very much leaning towards just sticking with my 50 1.8 S but now with Jordan and deadwolfbones both strongly advocating for owning the Viltrox 56 1.7 as well as the 501.8 S I am reconsidering.
I will aim to set up some good comparison shots to directly compare the two lenses. I really like the diminutive size and light weight of the Viltrox and, if there is little, if any, optical compromise that is quite compelling.
The larger and faster but heavier alternatives recommended above are intriguing but not what I am looking for at the moment.
Maybe in the future!