Correct. The two hard problems are sustaining a reaction over a period of time and, given that first constraint, having that reaction generate more collectible

Correct. The two hard problems are sustaining a reaction over a period of time and, given that first constraint, having that reaction generate more collectible energy than is being used to sustain it. We solved the problem of using fusion to create vast amounts of energy in a short amount of time half a century ago, but the primary application for that approach is excavation (and all ways that word can be applied euphemistically), not energy production.