What is the correct way to create a site via Config as Code (CaC) and use an existing repo?

what is the correct way to create a site via CaC referencing an existing repo and not be forced to specify a template repo?

Config:
{
  "$id": "grasp-junkman-washbowl-46e99",
  "$schema": "https://schema.yext.com/config/pages/site-config/v1",
  "siteName": "Answers",
  "repoConfig": {
    "targetRepoName": "answers.thisrepoalreadyexists.com",
  },
  "domain": "answers.mynewsite.com",
  "publishingConfig": {
    "pinnedStagingBranchName": "refs/heads/master",
    "pinnedProductionBranchName": "refs/tags/2-16-22",
    "stagingCredentials": {
      "username": "test",
      "password": "test"
    }
  },
  "features": []
}
Error: Could not create site. A template repository must be specified: Answers
  at /default/pages/site-config/grasp-junkman-washbowl-46e99.json

It is not possible to create a site in CaC using an existing repository with the site-config resource. Applying a repoConfig will attempt to create a new repository from a template repository rather than link to an existing one and will fail if that repository already exists.

So then would I omit the repoConfig key then or is that also invalid?

Yes, omitting the repoConfig will work.