One thing I don't quite get is the 20% soft-limit I've seen referred to - does this mean author rewards stop being paid out at 20% and then something else happens at 30%?

From vague memory:

• HBD stops being printed at the soft cap (now 20%). At that point author rewards would be paid in Hive (vested / liquid).
• At the hard cap (now 30%) HBD stops being pegged to $1. So on conversion you get $1 minus "the haircut". HBD then tends to move more in line with the price of Hive.

So one risk is that the volume of HBD expands during the bull market. And then if the market cap of Hive crashes in a bear market the 30% limit can be hit. So HBD may not be a safe-haven in a bear market.

At least that's what I remember. I haven't looked at it in a long while.
So not financial advice etc!