Resetting entire repository to a single commit:

git checkout --orphan newBranch
git add -A  # Add all files and commit them
git commit -m "first commit"
git branch -D main  # Deletes the main branch
git branch -m main  # Rename the current branch to main
git push -f origin main  # Force push main branch to github
git gc --aggressive --prune=all     # remove the old files

And additionally check remote branches: git ls-remote or delete them through the UI. And use: git push origin -d branch-name to delete those.

From: https://stackoverflow.com/questions/9683279/make-the-current-commit-the-only-initial-commit-in-a-git-repository