I had same issue with ghost images.
Local image cache cleanup - LXD - Linux Containers Forum
In your script, you can save two loops, by directly fetching base image:
output format: -f csv
—c, --columns: f for base image short, F for base image long
lxc ls --all-projects -f csv -c f
36acdd435250
47346cd1c1de
200424ff2f05
b3493d50c3da
lxc ls --all-projects -f csv -c F
36acdd435250cf6ec94cfefab3988ccd5fb642a9b297dfddd32f6e9ef17c0914
47346cd1c1def5e2a894518438c2d94f719baf37498f1bd86d1019cad94fb132
200424ff2f05b1c6a4c8a36b2394a6cc77f390222ade1bcad032df9982554518
b3493d50c3da36e38aa2268f91f66f64ad23c560e19998720faf270bda81adc8
f4c6939c4824ad15ffb9882464316ce3aa7f4ac95852ad7b0996a873eba818bf