I got to 8b second try. Get to the most valuable property as soon as possible and then ignore everything else not worth you time/clicks. Then buy as much as you can early in the Boom when prices are lowest. When a Recession starts, start selling everything while the prices are still high. Rinse and repeat. I kept all the debts until the end.

This only really works because the boom/bust cycle is too predictable and consistent, so it really just becomes a 'how fast can you click' game. It would be better if a) more random boom/bust points, duration, and rate of rise/decline, and b) buy and sell in bulk.

I've only played through once, but got 1.66B and felt I could probably get higher pretty easily.

My strategy was to initially buy high/sell low to get net value up so I could purchase higher value properties. By the time I could buy the most expensive properties I had stopped selling anything. Whenever a recession ended I would simply buy as many of the most expensive thing possible before the price raised too much. Similarly I would cancel any debts, and do any maintenance as soon as the bottom of the recession hit. Buying properties seemed to cancel the need to refurbish, but I haven't looked at how complex the logic is so I'm not sure.