There was a "green thread" experiment for dotnet a while ago, here is the conclusion: https://github.com/dotnet/runtimelab/issues/2398
But that's not how you progress.
Having proper M:N parallel runtime is mandatory these days and it baffles me that there people who are still not sold on it. I take it they have jobs and tasks where it isn't necessary but it's an interesting question to ask: for how long?