Disclaimer: I'm one of the creators of linq2db and extension linq2db.EntityFrameworkCore

linq2db itself has no dependencies to NetTopologySuite so library should know how to convert such types. Configure them ONCE per application (I hope covered everything):

var writer = new NetTopologySuite.IO.SqlServerBytesWriter() { IsGeography = true };

MappingSchema.Default.SetConverter<Point, DataParameter>(p => new DataParameter(null, writer.Write(p), DataType.Udt));
MappingSchema.Default.SetConverter<Polygon, DataParameter>(p => new DataParameter(null, writer.Write(p), DataType.Udt));
MappingSchema.Default.SetConverter<GeometryCollection, DataParameter>(p => new DataParameter(null, writer.Write(p), DataType.Udt));
MappingSchema.Default.SetConverter<LinearRing, DataParameter>(p => new DataParameter(null, writer.Write(p), DataType.Udt));
MappingSchema.Default.SetConverter<LineString, DataParameter>(p => new DataParameter(null, writer.Write(p), DataType.Udt));
MappingSchema.Default.SetConverter<MultiLineString, DataParameter>(p => new DataParameter(null, writer.Write(p), DataType.Udt));
MappingSchema.Default.SetConverter<MultiPoint, DataParameter>(p => new DataParameter(null, writer.Write(p), DataType.Udt));
MappingSchema.Default.SetConverter<MultiPolygon, DataParameter>(p => new DataParameter(null, writer.Write(p), DataType.Udt));

Then you can use BulkCopy for any entities with any Geometry property:

context.BulkCopy(someEntities);

Disclaimer: I'm the owner of Entity Framework Extensions
As you probably know inserting data into a table the "Entity Framework"-way is incredibly slow

That's true and this is the main reason why we created our library (paid library).

Our library support context.BulkInsert through Entity Framework and supports SQL Geometries as well.

That being said, this is also possible to do it directly through SqlBulkCopy .

For EF Core, you need to convert your value using a SqlServerBytesWriter .

Here is a full example:

var list = // your list

var byteWriterGeometry = new NetTopologySuite.IO.SqlServerBytesWriter();
var byteWriterGeographgy = new NetTopologySuite.IO.SqlServerBytesWriter() { IsGeography = true };

var dt = new DataTable();
dt.Columns.Add("Geometry", typeof(object));
dt.Columns.Add("Point", typeof(object));

list.ForEach(x =>
{
    dt.Rows.Add(byteWriterGeometry.Write(x.Geometry), byteWriterGeographgy.Write(x.Point));
});

var connection = new SqlConnection("your connection string");

connection.Open();

var bulkCopy = new SqlBulkCopy(connection);
bulkCopy.DestinationTableName = "your table name";

bulkCopy.ColumnMappings.Add("Geometry", "Geometry");
bulkCopy.ColumnMappings.Add("Point", "Point");              

bulkCopy.WriteToServer(dt);

The key for bulk insert with NetTopologySuite entities (of type geometry, geography) is:

• Set your datatable's column type to object (or your entity's property type to object)
• dtTable.Columns.Add("Shape", typeof(Object)); //force object
• Use the NetTopologySuite.IO.SqlServerBytesWriter() object to convert your shape to a byte array (byte[]) and set your object column / property to this byte array
• var byteWriterGeometry = new NetTopologySuite.IO.SqlServerBytesWriter(); dataRow["geometry"] = byteWriteGeometry.Write(shape);

Here's a dotnetfiddle using the fantastic Z.BulkOperations library which demonstrates this:https://dotnetfiddle.net/DZiQJG