How would cheap, accessible nuclear fusion realistically play out?

I find it hard to believe.

The easiest kind of fusion (D+T) by far uses Deuterium and Tritium and produces copious quantities of neutrons, more than either a thermal or fast fission reactor. There is a certain thickness of shielding you need to stop the neutrons and it's not small. Handling tritium is also a pain because it is an isotope of hydrogen that will go between the atoms of many metals, get incorporated into water supplies, etc. If somebody is handling tritium at energy-production scale you will always hear about leaks to the environment even if these are "mostly harmless."

Other kinds of fusion are a lot harder. The difficulties that drive fusion reactors like ITER to be huge would drive them to be even larger. I'd imagine interstellar travelers somewhere have huge D+D reactors so they can "live off the land" on Oort cloud objects.

How advanced a civilization is is strongly correlated to energy consumption. If cheap fusion energy is possible, then I think we'll see the next "industrial revolution" as many activities that were previously deemed infeasible for mass consumption due to energy costs will be possible.

e.g. Every household can have indoor greenhouses for home grown fruits/vegetables