I don't see myself using a non open source solution for such a critical part of my app Why not open source it and use an open core model?
If you are able to run and maintain a continuously integrated microservice on a kubernetes cluster (or something similar, maybe severless), then you really don't need Defer, as far as I understand it. There's already a lot of CI/CD and autoscaling open sourced.
I haven't tried it yet, but I guess most of the "runtime" is already open sourced, otherwise the background jobs can't run locally...