I learned a good tactic for apostle use from a Potato McWhiskey video
where he was working to rescue / rehabilitate a save file someone sent him.
Specifically to this question, he wanted to push back the religious influence of his next-door neighbor civ, on the same continent. Potato bought a pair of apostles with faith, and send them across the border together. When they conducted religious warfare with the enemy, the *overall* influence of the other religion was diminished. Not using their spread charges *directly*, but indirectly reducing the influence of the native religion. He mentioned that missionaries could come through after, doing direct spreads, to help clean up.
I've used this tactic successfully in a recent game as Mali (Mansa Musa). Keeping a pair of apostles near my border with Gilgamesh, I could fend off any apostles or missionaries he used to try to convert my cities. The indirect method also helps with city-states, where one cannot use inquisitors to remove the foreign religion.
Eventually, yes, you will need to send apostles to the other continent to convert those civs to win a religious victory. I would recommend securing your own continent first; this also depends a bit on which other civs founded a religion and where they are located. If most of the founders are on your landmass, then your efforts on other landmasses should be easier after converting your neighbors. Use the religion lens to see who your big opponents will be.
As you note, other civs who have a weak religion -- meaning that either they did not found one, or did found one but haven't spread it well -- will be a good place to start. You may find them to be a battleground between you and one of the other large religions.