Of course I am not stating that billions of death would happen overnight, like if we had a supervolcano explosion.

Instead, it would be gradual, over 20-25 years. But you are right that I am overstating things, when I write billions I think 'between 100 Millions and 1 Billion'. (Recalling that over 25 years, air pollution alone is responsible for around 100 millions deaths). It would depend on the exact temperature change: 3°C or 3.5C.

And total civilisation collapse in unlikely (I hope!), except if there is a feedback factor in global warming and we head for 4°C or more.