I don't have a strong opinion on formal business writing with LLMs because, in my experience, a shocking number of corporate managers are pretty terrible at expressing ideas or conveying information in writing. Frankly, for functional business report writing, in many cases I might find an LLM's rewrite to be more useful than the human written original.

In my experience, most formal business writing fell into two types: 1. Pro forma box-checking that probably didn't need to be written at all (or at least read by me, as a senior leader), and 2. Actually important information or ideas. The majority was Type 1, which I tried to avoid, and if it was Type 2, it was either well-organized and intelligible, or such a mess I'd have my admin schedule a meeting so the author could explain it and answer questions.

But I recently retired so I no longer have to suffer through formal business writing anymore :-)