You can use .NET string type instead of ObjectId, You just need to decorate it with BsonRepresentation. If you use BsonDateTime, you will have the same conversion issue. This is a domain class in my project that uses those decorators.
public class DocumentMetadata
{
[BsonId]
[BsonRepresentation(BsonType.ObjectId)]
public string Id { get; set; }
public string Name { get; set; }
public string FullName { get; set; }
[BsonDateTimeOptions(Kind = DateTimeKind.Utc)]
public DateTime DownloadTime { get; set; }
}