GitLab migrations seem to be hard to pull off and rather time-consuming, but I found this tool automated and transferred my organization from GitLab.com to my self-hosted instance rather seamlessly. What have others used?

Depends on what you mean by migration.

We tried enabling CI/CD for our repo on Gitlab but my account was in some weird shadow-ban state where I couldn't see the CI/CD settings page that Gitlab's documentation was alluding to. Consequently, we sought Github.

We didn't have any vendor tie-in to Gitlab and our migration was just changing the Git remote repo.

  └─ ▶ git remote -v
  origin  git@gitlab.com... (fetch)
  origin  git@gitlab.com... (push)
  └─ ▶ git remote rm origin
  └─ ▶ git remote add origin https://github.com/$USER/$REPO_NAME.git
  └─ ▶ git push

With CI/CD, we now have vendor tie-in. Given that it took us a few hours and a few attempts to setup Github actions, and Gitlab is likely on the same order of magnitude of effort, the vendor tie-in doesn't seem to strong.