I got advised to post this here for visibility: Support serverless runners · Issue #1030 · actions/runner · GitHub
Would be very valuable to have serverless self hosted runners in GHA. This way it would be easy to utilize platforms like Google Cloud Run similar. no resources would be wasted, it would be easily scaleable and the setup would be much easier. GHA would be a perfect fit for serverless environment since they are stateless and spun up based on event.
There are some community driven implementations for AWS: GitHub - philips-labs/terraform-aws-github-runner: Terraform module for scalable GitHub action runners on AWS but the setup seems to be so complex that it defeats the purpose especially for smaller teams.