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'