Here’s an update on my experience, perhaps it will help someone who’s in the same situation as me:
Starting from an install of Volumio 2.444 on a Raspberry Pi Zero W, I had the problem of no Volumio access point appearing even after a long (>15 minutes) wait after first boot.
What helped was to cut power to the Raspberry Pi,
pull the SD card out,
insert the SD card in a computer running Linux,
run GParted and point it at the SD card,
use GParted to perform a file system check on each partition on the SD card,
safely eject the SD card,
insert the SD card in the RPi,
apply power,
wait about 3 minutes.
The Volumio access point does appear. I’m able to attach to the unit using a browser. Successful in configuring and using Volumio from there.