I see that the white paper addresses this concern, but abuse still seems rather rampant. What challenges are you facing addressing tag misuse?

I would say that it's probably due to the lack of follow-through by team-members to report, warn and downvote offenders (I too am guilty of this).
I do not think it is entirely intentional though, as for me, there is often so much that needs to be done, that finding the time to do manual curation in addition to everything else has proven to be quite challenging, (and I get the feeling I'm not the only one.)

And how has HF21 impacted post promotion?

There were talks of a appreciation & downvote bot that was intended to deal with this, however since HF21, we ended up returning to the initial directive of manual curation (ie. through utilizing our post promotion channels in discord, or participating in our quests and submitting posts to be appreciated based on the community guidelines tier appreciation system.)

Furthermore, while the community guidelines were created in hopes of helping to alleviate the issue of tag abuse, existing team-members have expressed hesitance with downvoting spammers, due to the fear of retaliation on their personal accounts (which some have experienced upon downvoting offenders in the past).
However, perhaps there needs to be a team member who's job is specifically related to rectifying this, namely appreciating good posts and providing warnings/downvotes to spammers, (will have to consider/flesh out this option further.)

Something that is important for a tribe token is that there are built-in sinks

Some of the utility that has yet to be implemented includes -

• Prize pooled community run contests that encourage entrepreneurial blogging with the chance to win nfts (across various crypto-games), tokens, and sponsored novelty items from showcased businesses.

• We are also due to create another website where our services are listed and where we hope to make advertising available to interested crypto-based businesses.

Both of the above options have additional facets to them that I'd prefer to leave undisclosed until they've been discussed and finalized by the team,
but truly I'd have to say our biggest challenge has been learning to grow into the shoes of managing such a large project, with little to no experience prior to this.
Our team members also happen to be spread out across numerous other projects on the blockchain, which in addition to the differences in time-zones, has made for some particularly 'challenged' team meet-ups.
Still, despite the project's shortcomings at this time, I believe our obstacles are surmountable and genuinely look forward to improving on the quality of service that Intrepreneur provides, as we continue to learn and grow alongside our community.

I'd also like to thank you for your consideration and input in this matter, as it is greatly appreciated. :)