This is interesting 😋 Thanks for sharing it.

A way to measure build their brand could be Steem Power. You could include any non-vested assets in the Steem wallet also, as they are part of author rewards. Increase might matter too, though I can imagine this being debated against.

However maybe that just shows that increasing Steem Power, and thus influence, is the core action of Steemit.

This might be problematic because of course it doesn't apply outside of Steemit, which building your brand does, so there's pros and cons.

Side note, I don't think a core action needs to be reduced to something a button could do right? Every post could be seen as directly or indirectly writing about your brand or promoting your brand if it comes from your brand, i.e. your account, and thus any positive reflection on that builds your brand.