I moved /etc/syncthing/ to the USB drive and then linked it back. Didn't want to wear the flash out with all the writes, but I suppose it resolves the space issue you are seeing (did this before actually adding any big directories).
mv /etc/syncthing /mnt/sda1/syncthing
ln -s /mnt/sda1/syncthing /etc/syncthing
Haven't done a sysupgrade on this router, but I suspect it will whack the link, so look out for that.