Is there a way to restrict it to only signing in for existing accounts?

I am not currently aware of a suitable way to implement this.
You could restrict access with the SignIn callback, but you can't check if the user exists or not. There is currently an open issue on authjs/next-auth that focus your problem.

Is there a way to manually create a link, or to specify that an account with Id equal to the oauth account id will link.

Could you clarify your question or give an example?
You could create a database entry (user, account or whatever) directly via the payload api.

Is there a way to edit the sign in button, ideally, I'd like it to just be Sign in with

This is not possible at the moment. I will add the customization in the future.

By the way, I have added the option to change the text of the sign in button:

authjsPlugin({
 authjsConfig: authConfig,
 // Customize the sign-in button text
 components: {
 SignInButton: {
 text: "Sign in with <service>",
 },
 },
})