Is AFP of any value to non-ancient devices? From one person’s tests, SMBv3 is just a hair faster than AFP.

All of the Macs, iPads, and iPhones in our house capable of reading disks are Apple devices that support SMBv3.  We have no devices capable of AFP.

File sharing allows SMB or “SMB and AFP.”  Is there any benefit to enabling AFP?

I am more accustomed to NFS, but speed of disk access is worth switching over.
I personally use SFTP on my home network rather than SMB or NFS, as that allows you to access POSIX file permissions, whereas the others do not. This is a feature offered by AFP, too.

macOS requires POSIX file permissions on users' home directories, so if you plan on having those accessed via the network, SMB is not an option.

Having said that, Apple appears to be phasing out AFP, with no support for APFS volumes (at least not yet). I'd personally stick to just using SMB unless your needs dictate otherwise, in which case, SFTP may be a better choice than AFP simply for its cross-platform support and not being proprietary (though Netatalk provides an AFP solution for Linux).