What's the correct stock ROM for my unlocked Moto E5 Play?
I've figured it out. Besides the carrier-specific ROMs, Motorola has a "retail channel", called RETUS (Retail - US, I believe).

This is not tied to any carrier, and you can match the factory version by comparing with what you get here:

# fastboot getvar ro.build.fingerprint
(bootloader) ro.build.fingerprint[0]: motorola/james/james:8.0.0/OPP27.9
(bootloader) ro.build.fingerprint[1]: 1-121/42:user/release-keys


Combine the two lines, and we have 

motorola/james/james:8.0.0/OPP27.91-121/42:user/release-keys