If dark energy is truly a cosmological constant, then, as you say, we will approach a pure de Sitter space where the universe will be in thermal equilibrium (maximum entropy) and will produce thermal fluctuations. However, if the fundamental (quantum-gravity theory) allows flat spacetime (i.e. the true vacuum) as a state, then the maximum entropy is infinite (because there is an inifinite dimensional Hilbert space) and the universe will (eventually) approach flat space where there are no thermal fluctuations. (See https://arxiv.org/pdf/1405.0298.pdf for the details).
Also, we should not trust any physical theory that predicts (mostly) Boltzmann brains, because such a theory cannot both be true and justifiably believed (https://arxiv.org/pdf/1702.00850.pdf).
On the second point: I see Boltzmann brains as issues of decision theory, not probability theory, so I’m not worried about probability issues with them.
Some points I want to add to the discussion:
If dark energy is truly a cosmological constant, then, as you say, we will approach a pure de Sitter space where the universe will be in thermal equilibrium (maximum entropy) and will produce thermal fluctuations. However, if the fundamental (quantum-gravity theory) allows flat spacetime (i.e. the true vacuum) as a state, then the maximum entropy is infinite (because there is an inifinite dimensional Hilbert space) and the universe will (eventually) approach flat space where there are no thermal fluctuations. (See https://arxiv.org/pdf/1405.0298.pdf for the details).
Also, we should not trust any physical theory that predicts (mostly) Boltzmann brains, because such a theory cannot both be true and justifiably believed (https://arxiv.org/pdf/1702.00850.pdf).
On the second point: I see Boltzmann brains as issues of decision theory, not probability theory, so I’m not worried about probability issues with them.
https://www.lesswrong.com/posts/ZvmicfmGg9LWBvy2D/boltzmann-brain-decision-theory