I’m not sure which part I have configured wrong then?
I followed your config file and changed all instances of "/mnt/sda1/syncthing " to "/mount/mmcblk0/syncthing ".
config syncthing 'syncthing'
option enabled '1'
option gui_address 'http://0.0.0.0:8384'
# Use internal flash for evaluation purpouses. Use external storage
# for production.
# This filesystem must either support ownership/attributes or
# be readable/writable by the user specified in
# 'option user'.
# Consult syslog if things go wrong.
option home '/mnt/mmcblk0/syncthing'
# Changes to "niceness"/macprocs are not picked up by "reload_config"
# nor by "restart": the service has to be stopped/started
# for those to take effect
option nice '10'
# 0 to match the number of CPUs (default)
# >0 to explicitly specify concurrency
option macprocs '0'
# Running as 'root' is possible, but not recommended
option user 'syncthing'