This Action is suitable if you:

• are maintaining a fork
• have changes that are not going to be merged into upstream
• want to keep changes based on the latest upstream

Basically this is doing git rebase upstream master. If there are conflicts, it simply fails.

# .github/workflows/sync.yml
name: Rebase Upstream
on:
  schedule:
  - cron: "0 0 * * *"

jobs:
  sync:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@master
      with:
        fetch-depth: 10  # larger than the number of commits you made
    - uses: imba-tjd/rebase-upstream-action@master
      id: rebase
      with:
        upstream: https://github.com/<user>/<repo>.git
    - uses: ad-m/github-push-action@master
      if: steps.rebase.outputs.needs-push
      with:
        force: true
        github_token: ${{ github.token }}

• tgymnich/fork-sync and apps/pull: I don't want PRs. Besides there is not way to do a git rebase on GitHub website
• repo-sync/github-sync: It's not using rebase or merge. It completely mirrors the upstream. And it can't sync current branch
• wei/git-sync: Very complicated and have the same issue as github-sync. After all its aim is "syncing between two independent repo"
• This one: Not widely tested. Use with caution