> What would trying to sell billions in stock at once, or leverage hundreds of thousands of rental properties look like to the larger economy, and what would the effect be?

Billionaires already routinely sell billions in stock "at once" (meaning, per quarter or similar, not a $1 billion limit order on Robinhood...), so on that one, we can empirically suggest "not much of an effect on the larger economy".

Randomly chosen examples:

https://finance.yahoo.com/news/bill-gates-liquidated-1-7-180...

https://www.reuters.com/business/autos-transportation/elon-m...