Coal contains between 1 to 10 ppm of uranium. Globally about 8 billion tonnes of coal is burned every year to generate electricity. Thus between 8 thousand and 80 thousand tonnes of Uranium is emitted into the atmosphere every year.

Which… I’m not sure, but that might be more uranium than would be needed in the first place to run our entire civilisation on nuclear power?

Certainly it’s in the ballpark.

Just current reserves, with breeder fuel cycles, would allow us at least a century, long enough to start building in space...

It's not a a useful measure, since no one would collect uranium in this way but assuming someone would. Some napkin math... 27 tonnes per 1GW reactor/year is used and there are about 440 reactors so about 12kton.

MW by MW, coal plants emit more radiation than nuclear on average. This is true even when taking accidents into account, and not by a small factor.

We’d genuinely be better off ignoring most accidents and just using nuclear. Not to mention that coal ashes and emissions are chemically carcinogenic as well.