Thank you very much for working so hard.

About the 'working so hard' above, does it only mean a completed action, no matter what context? Or in certain context, it could be an action in progress?
I guess it can always have two meanings.

You worked hard. Thank you for working so hard.
You are working hard. Thank you for working so hard.

Or at least I hope so. Emotion: smile
In theory, Thank you for having worked so hard is ok too, but most people just say for working instead.
Kooyeen is correct. It can mean both a completed action or an action in progress.

You could also say, "Thank you very much for your hard work."