I've been told the Android One program is the spiritual successor to the Nexus line. Does anybody with more knowledge or experience feel like commenting on that? Do those phones get updates as reliably as a Nexus would in the past?

Also, how does one shop for a phone these days when you want plain old Android and sensible security updates but have other feature concerns as well? I don't want a flagship phone, don't want a giant phablet, and care more about offline GPS performance than data performance...

I am still carrying a Moto G4 Play which I think is borderline too big. It still lasts almost a week on one charge when I take it into the woods in airplane mode and just use the camera app and OSMAnd+ with GPS logging for 3-4 hours per day.

As I understand it, the Android One phones all receive updates from the vendor and not directly from Google. Historically vendors have not done a good job at making updates available in a timely manner.

Google handles the updates for Android One. I have a Xiaomi Mi A1 in addition to a Pixel and it seems to get updated on time. Occasionally, faster than the Pixel even for security updates.