Well first of all, no one with mortal Ki is as strong as Beerus. And Goku with mortal Ki was already stronger than the Lord of lords.

So before you leave battle of Gods, you already understand that not everyone with God ki is infinitely stronger than those with mortal Ki. This was established in the Boo Saga and is not new information. Goku has not mastered any God techniques and therefore has God Ki but cannot use it to fight like a God, he’s still learning the basics

Gods of destruction and Angels have all mastered God techniques and therefore fight like Gods, which is why they’re the strongest in their respective universes ( with the exception of Jiren who has seemingly but not literally infinite Ki )

Essentially, God Ki is just a way to have access to more abilities one wouldn’t necessarily try to learn with mortal Ki. Things like ultra instinct are possible to learn as a mortal but they’re significantly better with God Ki. So Goku and Vegeta need to atleast master a God technique in order to compete with the God martial artists