export one stream from simulcast

simulcast -> createPlainTransport() -> ffmpeg
ffmpeg records the most simple stream (scaleResolutionDownBy: 4)

Is there a chance to tell plain_transport.consume() to export only the best stream?

This is not true. By default a consumer will take the highest stream from a simulcast producer. Then there is an API in Consumer class to select which stream to consume (setPreferredLayers).

So there is something wrong in your setup.