Check this out. This was at one point one of the cheapest and smallest Linux computers around. It’s USB powered and this project turns a WiFi device designed to share photos from an SD card over a standalone SSID into a male USB A powered miniature SBC. (Edit: okay it’s two PCBs technically)
https://github.com/Emeryth/openwrt-zsun
https://wiki.hackerspace.pl/projects:zsun-wifi-card-reader
I got them in bulk from China for ~$6 each.