Let me start with those three points as general overview of my stance on the matter:

1. I'm not really convinced Hive has problem with inflation.
2. HF28 reduced base for inflation calculation quite a bit and it is too early to assess the impact.
3. Inflation is irrelevant when people have reasons to buy in and stay invested in the chain, so instead of fighting inflation, giving people those reasons might be a better option. The last thing we want is to give people reasons to leave Hive.

Also, I'm writing this comment on the fly as I'm reading your article, so some points might turn out to be already covered.

Witnesses should consider temporarily reducing HBD APR to ~6%

Absolutely ridiculous. It is exactly the opposite of what should be happening when price of Hive is low. It is already critically hard to justify keeping HBD in savings right now:

1. Hive is not in a vacuum. When you have 1k HBD it might not make sense to constantly look for options, but when you have 1M that's a different story. When deciding on APR it is important to compare with alternatives, mainly all the asset backed stablecoins on the market, that have liquidity orders of magnitude higher, they might soon start to be directly accepted for payments (or maybe they already do), there are options allowing for gaining APR on them. Treasuries and similar traditional products should also be considered.
2. HBD already does not offer real backing with Hive that would allow big investors to get back 1$ per HBD. With 1M HBD you'd get 10M Hive from conversion (forget internal market) and that's a all of Hive's trading volume for a week on Binance. Realistically you'd need to spend several weeks trying to liquidate such amount without immediately crashing the price. With that in mind a real alternative to keeping HBD is to convert it to Hive and stake, gaining passive APR from VESTS and automated curation (just bot-vote for stabilizer - it should be safe enough from downvotes). I'm not sure how much it gives after HF28, and the curation part depends on vote competition (low price of Hive usually entails less voting activity which increases gains of those who keep voting) but both sources combined used to give about 8% APR on top of all other benefits of staking.
3. If you think Hive price will tank, keeping HBD as protection against that is very risky so close to debt limit, meaning it is better to convert now and sell (staking in the meantime to cover some part of the risk with staking/curation APR while you slowly liquidate). If you think current low price is temporary, it is better to convert and stake, because upside potential far exceeds 15% APR on HBD.

You can't reduce HBD APR to 6% because even within Hive ecosystem there are alternatives, unless of course the effect you want to achieve is to permanently discourage people from keeping large sums in HBD, or you also plan to destroy the alternatives giving people more reason to abandon the chain.

curb overspending and DHF outflows

My stake is far too low to ever take a deeper look in what is actually financed, but I know that most of development funds don't come from DHF, but should. Also the way DHF payouts are structured is incompatible with real world requirements, like f.e. it is a criminal liability to stop paying your employee when DHF stops paying you, so you need solid buffer which increases cost.

note that we are aware that this may cause conversions to happen

So you want to make sure they happen, ok...

The DHF is another major source of new, non-programmatic inflation.

Not entirely true. All funds in DHF come from regular inflation (well, except of ninjamine, but that's already lowered quite a bit). We are heavily underspending since forever due to return proposal, plus the funds are kept in HBD, which with low HIVE price made treasury stash quite big, so the allowed spending is also relatively big. It would be interesting to simulate what would happen if the funds were always kept as Hive and only converted to HBD on payout - how would that affect how much we can spend each hour (I suspect in the end it would just mean less funds returning through return proposal, but who knows).

Since there is no built-in cap on DHF outflows

There is, technically we are always spending everything that the cap allows, it is just that return proposal gives part of it back to treasury.

Add the Value Plan's annual 1M HBD

I think I'm missing something - what is a Value Plan and how it is financed if not through proposal?

Examples of Deliverables & KPIs We Want to See

I was under impression DHF was supposed to be used mainly for R&D, which in every organization represents a cost, hardly ever attributable to specific profits, it just allows organization to stay afloat on the market in the long run.