On a timescale of 1e11 years, all uranium and thorium will have decayed, but planets will still exist, orbiting red dwarf stars.
Would it be possible to use lead as a nuclear fuel? In principle, the curve of binding energy says lead fission is an exothermic reaction. Is there a way to do this while getting out more energy than you put in?
(Getting out more money than you put in, is of course a bigger challenge, but we are talking about a scenario where there is no competition from uranium or thorium let alone fossil fuels. I'm assuming the minimum size for a viable fusion reactor remains large compared to a few gigawatts.)