At this moment author/curation rewards are distributed between all hashtags equally. We are debating to count first hashtag as 2X.

the domain expertise is a reflection of tag usage frequency and driven by the post payout when using these tags?

This is accurate.

As you may know, Waivio supports wiki-listings/lists/pages on Steem blockchain - we call them objects. And when you submit a post through Waivio, you have an option to edit assigned weights:

Originally it was designed to allow authors to avoid adding weight to unwanted objects, but it can be also used to focus on acquisition of domain expertise in a specific object.

We might need to add a similar edit tool for hashtag references as well.