The problem of selecting text inside a link in Webkit/Blink engines has existed for a long time. At least for me. Somehow I missed that it could be solved with a browser extension. I found such an extension, called Select like a Boss. But it was only for Firefox, Opera (Blink), and Chrome. Since the source code for the Chrome extension was available on GitHub, I simply took the js code and packaged it as a Safari extension. Now I'm happy and can properly select text inside links :)
Source code + download link: GitHub.