I had looked into this a while ago, and I forget why, but I thought it was a mismatch; however, looking again at the list of downstream projects, this one at least seems compelling for the use case I described:

https://docs.earthly.dev/examples/monorepo

At least it addresses building the right Docker image--it doesn't seem to address building the broader Kubernetes manifests, but I'll take what I can get at this point.