The SOC used in the Fairphone 5 will receive 5 years of support from Qualcomm this will also benefit custom OS'. And the FP5 isn't just about longer support it's also about repairability, sustainability and good working conditions. Yes it's expensive, but so are other high-end smartphones that are even more expensive and only last 2 to 3 years.

Probably through Murena like the Fairphone 4

Fairphone doesn't distribute spare parts to the US but Murena does.