Actually I have not found any of the built-in Actions to be completely reliable to tell me exactly when a web page has completely finished loading. So, I wrote this macro:
MACRO: Pause Until Web Page Element Exists [Example]
This works with both Safari and Chrome. The key is identifying a web page HTML element that will exist when you need to start using the page.
I have NOT tested this macro with Safari 12.1, nor under Mojave, but I believe it should still work fine. If not, please let me know.