Mongodb supports EPSG 4326 by default you can specify a custom crs : https://www.mongodb.com/docs/manual/reference/operator/query/geometry/
My workflow is simple : geopackage to mongodb.
I tried :
- Specify source and destination projection : no effect (data are in 4326 in mongo)
- Extract geometry (projected in 2154) and store in attribute geometry2 but there is no json type (or even geometry type).
- Extract geometry and write it in a geometry attribute : fme returns an error.