Would expanded civilian nuclear power provide some kind of sustainable helium production as a side effect or do the numbers not work out?

Only if we change to fusion power, which is just fifty years away.

Even nuclear fusion doesn't produce helium in large amounts; a large fusion plant would produce about 2000 liters / day, meaning you'd need 136 years worth of output from one station to fill a single airlander airship.

Same goes for fossil fuels, but that doesn't stop us from burning billions of tons of them per year like we have an infinite supply.

And unlike fossil fuels that are (likely) only found on Earth, Helium is the second most common element in the universe and can be found literally everywhere once you're off planet. Plus we can technically manufacture it from radioactive decay.

> can be found literally everywhere once you're off planet

In the vacuum of space, not enough to be harvested. Presumably the same for asteroids, moons and planets.

Stars have lots and lots of helium, but good luck mining it.