I feel like the most likely sort of collapse would be a Roman empire style collapse where it takes centuries to reach completion and we see a period of increased governmental instability/local authoritarianism for a little bit. The most likely cause would be some sort of climate disaster. It probably wouldn’t happen everywhere either, the Byzantine empire lasted well into the middle ages, after all, going on the Rome metaphor. The best strategy would be to move somewhere less effected by the collapse with a hospitable enough climate to support local food production, and enough resources to ensure long-term maintenance of infrastructure. The Great Lakes area of the US/Canada fits this pretty well