I would think that using fusion or fission for synthesizing elements is going to still be less efficient (among all resources, not just energy) than using the newfound abundant/cheap energy to extract those preexisting elements from mixtures that exist on Earth.

Take neodymium, your example. That's pretty abundant in the Earth's crust. It's just that it's energy intensive to extract it from the mineral formations that naturally occur. At that point it's still probably much cheaper, energy wise, to separate a bunch of minerals into their constituent elements, rather than try to synthesize atoms through fusion and fission.

The specifics are a pipe dream but the general principle holds: if energy suddenly becomes more plentiful and cheaper by orders of magnitude, society will find a way to use that new plentiful resource in ways that we can scarcely imagine today. That's always been true of new inventions, where much of the post-invention innovation comes in the form of finding new applications for a thing that has already been invented.