Have you tried cloud run on GCP? It sits in the niche you're describing between a serverless platform and some managed container orchestration platform like kubernetes (GKE or EKS).
https://stackoverflow.com/questions/55786955/whats-the-value...
The computer science part of me just looks at a Docker swarm as a big graph. We should be able to balance a load if we just know the remaining CPU capacity of each container. But I look at the astonishing complexity of all this stuff (not to pick on k8s too much) and my first thought is: never have I seen so much code do so little!