Even if they don't go completely dormant, I'd think they slow down during the cooler season. That would be the best time to prune it. If you are worried about it weeping and loosing energy, treat the cuts with tree wound dressing. If you are repotting a size up you don't really need to root prune. Couldn't you just put it in the ground? That may be the best solution for where you live.

My ground soil is very poor I know that figs produce fruit even at poor soil but still it is super poor and the container is really thick and I think getting it out of the container JUST for pruning it will be really hard work every year or two

HU,

root pruning is a must when growing in a container as eventually the roots will start to circle and slow strangulation occurs eventually killing the plant.

You can wait until after the ripening crop and watch the leaves for any change as in color or some dropping then wait until soil is fairly dry yet compact enough not to fall apart, tip container to side grab trunk and pull out.

When root pruning you must top the canopy as well otherwise you risk plant dropping figs because to much canopy the pruned root system cannot support and plant will compensate by dropping figs thats mother nature helping its survival.

Martin