You do not need a case for that! The NVMe heats up on its own. There are a bunch of reviews going into that topic, because it just matters for high end NVMe drives. See for example https://www.computerbase.de/2020-09/samsung-980-pro-ssd-test... - though the 980 Evo did well without an additional cooler, and here it's indeed depending on the airflow.

You seem to assume it's the case that heats up to 80C or more, that's not true and also not necessary for NVMe SSDs to go over their limit. Your CPU/GPU has fans moving the heat away, that's a better position to be in...

How is the lifetime of a SSD run around 70° vs the same SSD with a slightly bigger heatsink that runs at 60°?

I would run some tests though.

There are all kinds of vendors / models, who knows what kind of throttling they use?