Hi Ray,
Thanks for the update.
Currently, we do not have direct support for loading the data of LineString type from the JSON file. We will check whether we can consider this as a new feature request to our SfMaps widget.
However, if your requirement is to achieve an UI similar to the below image, we will check whether we can achieve this using a workaround with the current APIs itself.

https://help.syncfusion.com/flutter/maps/vector-layers/polyline-layer#adding-polylines
Could you please confirm whether you are trying to add the lines as a sublayer on top of another shape layer as shown in the above image? If not, we kindly request you to provide more details or an example image of it.
Regards,
Lakshmi R.