Buy a used Nissan Leaf, then?

The sunk cost fallacy applies not only to dollars, but also to any quantitative phenomenon. The important thing to evaluate is the cost going forward. When you look at the cost of that new Nissan Leaf, you need to amortize the initial carbon cost over the rest of its lifetime, not just the few years you have it!

From an old forum post in 2013, the best I can find is about 15 tons.

I suspect it's much lower than that number now-a-days. Primarily because the energy going into batteries production will both use less power and likely comes from greener power sources.

If nissan is to be believed, since then they've cut the CO2 emissions from production by 40%. So maybe in the range of 9T?

I kept my last car for almost 20 years for that reason, but parts were rusting off - the fuel tank fell off was what made me give up. At this point that car is scrap and I am in a newer car that is made no matter what, so that co2 is a given.