The buried lede on those stories is they will not spend out of their FIRE budget on activities to bring that fulfillment, leaving them with somewhat limited choices depending upon their personalities and proclivities (see below), and many have not even planned for how much that will cost.

There are very few personality types that would be happy as a clam spending a hundred years puttering around the stacks of libraries researching a particular scholarly pursuit, then spend another hundred on another specific area, for example. The list of what we do not know is practically infinite, and were I immortal, I could burn a few thousand years researching subjects I find fascinating. Cheap for someone on even lean-FIRE.

There are still quite a few areas of the world that are very sparsely-mapped and their microbiomes sparsely-cataloged to help preserve their bio-diversity. I could spend quite a few centuries just doing that, while hiking and camping. Relatively cheap as pursuits go.

Refurbishing and repairing old artifacts would require some spending on tooling and supplies, but still cheaper than say, learning to fly. I could spend a few thousand years just learning different trades to master craftsman skill levels.

I find the RE experiences others relate that they feel aimless baffling, but only due to my personality. So it is not surprising to me that different personalities cannot conceive satisfaction within the same pursuits. Pursuits that could cost far more than they realized they needed to budget for. FIRE is more about knowing your self than hitting an appropriate total that survives a 2% SWR.