Here is a Plug In Action that performs similarly to the Wait For Safari to Finish Loading action. It is not as good, since the Stop menu used to be quite helpful in determining whether a page was loading, but since that is no longer the case in Safari 12.1 this will have to do.
It is pretty straight forward, but I have not done much testing of it. Hopefully it works - if any AppleScript gurus (@ccstone?) want to look and check I have not messed up my AppleScript coding, that might be wise.