Thanks!
I gave this a try, running LXD as a snap.
It appears that it cannot find the character device, and it might be related to the fact that /dev/serial/by-id/usb-Silicon_Labs_CP2102_USB_to_UART_Bridge_Controller_0001-if00-port0 is a symbolic link?
$ ls -l /dev/serial/by-id/usb-Silicon_Labs_CP2102_USB_to_UART_Bridge_Controller_0001-if00-port0
lrwxrwxrwx 1 root root 13 Σεπ 28 14:59 /dev/serial/by-id/usb-Silicon_Labs_CP2102_USB_to_UART_Bridge_Controller_0001-if00-port0 -> ../../ttyUSB0
$ lxc config device add esp32 ttyUSB0 unix-char source=/dev/serial/by-id/usb-Silicon_Labs_CP2102_USB_to_UART_Bridge_Controller_0001-if00-port0 path=/dev/ttyUSB0 major=188 minor=0 uid=0 gid=20 mode=666
Error: not found
Exit 1