I agree with "Symbols" being a confusing name for it. I thought it would actually be an actual symbol. We can brainstorm with the idea for a new name, such as "Reusable Elements", or maybe "Copy-Cat" if you're looking for a fun name.
As for the indicator, that would be very useful. I already like the Diamond Logo, maybe if there's different logos per element? Or for it to be easier, it can be the diamond logo, but different colours for the indicator, so we won't run out of colours easily.
Also I noticed when you "unlink" a symbol, the symbol logo still stays in the layers and in the element itself. Just a small bug report.