But looking at the Batocera files, I now realise I am definitely doing something wrong.
I thought I was creating a link to the userdata/share/saves/snes folder but clearly I am not, it’s just creating a folder in userdata called HX99G Syncthing.

I have now tried calling the folder Syncthing, no joy.
Tried working from the router to the Batocera machine, no joy.

Every attempt results in the same error message.

root@GL-AXT1800:/mnt/mmcblk0/syncthing# ls -l
-rw-r--r--    1 syncthin syncthin       794 Jan 27 14:49 cert.pem
-rw-------    1 syncthin syncthin      7901 Jan 27 15:27 config.xml
-rw-------    1 syncthin syncthin      7274 Jan 27 14:49 config.xml.v0
-rw-------    1 syncthin syncthin        33 Jan 27 14:51 csrftokens.txt
-rw-r--r--    1 syncthin syncthin       798 Jan 27 14:50 https-cert.pem
-rw-------    1 syncthin syncthin       288 Jan 27 14:50 https-key.pem
drwxr-xr-x    2 syncthin syncthin      4096 Jan 27 15:38 index-v0.14.0.db
-rw-------    1 syncthin syncthin       288 Jan 27 14:49 key.pem
root@GL-AXT1800:/mnt/mmcblk0/syncthing# cd syncthing
-ash: cd: can't cd to syncthing: No such file or directory
root@GL-AXT1800:/mnt/mmcblk0/syncthing# cd /mnt/mmcblk0/syncthing
root@GL-AXT1800:/mnt/mmcblk0/syncthing# ls -l
-rw-r--r--    1 syncthin syncthin       794 Jan 27 14:49 cert.pem
-rw-------    1 syncthin syncthin      7901 Jan 27 15:27 config.xml
-rw-------    1 syncthin syncthin      7274 Jan 27 14:49 config.xml.v0
-rw-------    1 syncthin syncthin        33 Jan 27 14:51 csrftokens.txt
-rw-r--r--    1 syncthin syncthin       798 Jan 27 14:50 https-cert.pem
-rw-------    1 syncthin syncthin       288 Jan 27 14:50 https-key.pem
drwxr-xr-x    2 syncthin syncthin      4096 Jan 27 15:38 index-v0.14.0.db
-rw-------    1 syncthin syncthin       288 Jan 27 14:49 key.pem
root@GL-AXT1800:/mnt/mmcblk0/syncthing# whoami
root

ls -l is reporting that Syncthing is the user.
whoami reports as root.
Is that because whoami is referring to the device as opposed to the /mnt/mmcblk0?