The external treasury idea is okay, but a lot more complicated that in sounds to implement in practice.

I would disagree on this point:

Adding interest means creating inflation out of thin air for HBDs (if I'm wrong correct me on that, it would make my reasoning invalid). So we are going closer to the debt ceiling, much faster than in the previous bullrun, especially if this is successful and we raise interest rates even more

I don't think even higher interest rates are particularly significant here. It would take many years to increase the HBD supply by enough to matter, and that is assuming that over that period of years the increased supply wouldn't be offset by other factors (most clearly decline in demand for HBD as its supply gets too large), which is probably false. And as I mentioned in the post, a growing level of HBD is still sustainable for a very long time if not permanently, as long as the value of HIVE grows even faster. In the short term this is unreliable, but over a period of many years it is a reasonable expectation as long as Hive doesn't just fail (in which case HBD doesn't matter either).

I also think the 10% debt ratio cap should be increased. It was introduced hastily (by Steemit) without much consideration or discussion. Even in the original white paper. I believe debt ratios of 20% or 30% were mentioned as being okay, and that might even be low.

Under most conditions the market should dictate how much HBD is sustainable, since if there is too much and it is insufficiently backed by value of the underlying token, its value would drop. A cap is perhaps needed to prevent frothy "bubble" market situations from far exceeding what a more sober analysis would support, but it doesn't need to be 10%.