Space

True! Long-term travel within the heliosphere is still thwarted due to the radiation of CMEs which require 350cm of concrete for protection. (Our manned trips to the moon were timed during solar minimum, and still had some worrying moments). While a moon base would could be accomplished by putting it underground (or having shelters underground at minimum) it still keeps us from getting to Mars.

Then there's the matter of creating a self-sustained ecology. All of our efforts so far have either died off or required infusions of elements to perpetrate. Also only a matter of time before we work out a configurable system.

But both of these are something like nuclear fusion, in that we know it can actually be done because we have natural examples (even if self-sustaining nuclear fusion only exists in the core of a star, we're pretty sure it happens IRL). We don't have signs of FTL or megastructures that don't collapse into a giant ball of mass. To qualify for a megastructure, we'd simply have to create something that is millions of meters. Our current freestanding structures measure in the hundreds of meters, and we have at most a few kilostructures like the LHC (27KM) which depends a lot on its fixture to the earth.

So yeah, it's a matter of developing the technology, but I suspect there are a lot fewer decades and great filters between when we figure out surviving CMEs in space versus building megastructures in space that don't collapse.