I am already using GeoJSON.NET and I am planning a migration to System.Text.JSON. But I also serialize to other formats, e.g. Mongo-BSON.

What I don't understand is the coupling to the serializer. The fields with GeoJSON objects are part of my domain and I think it would be great to have it decoupled from the serializer. GeoJSON is so widely used that it is almost the standard for all representation of geo objects.