I would rather not call a big freeze a case where a single subatomic particle is left, but more a gigantic universe where every single particle is isolated.
I have the impression that what you have in mind is instead a kind of soup of elementary particles like in the first moments of the universe. This would correspond to a big crunch scenario. In this case, the problems of the first fractions of second are the problems of the last fractions of second too. As there is no more time defined in there, we can think long about them :D
Yes, my bad. I actually was thinking of a soup like state (had bose-einstein condensate like state in mind, hence why I said that energy is so tiny, but yet subconsciously thinking fraction of a Kelvin is beyond freezing. So I typed "big freeze" 😁)
I see. It is indeed what I imagined ;)