You might store the ObjectId.ObjectClass.DxfName instead.
I need to look this up or read back up on it but if your dll is not in the same folder as the acad.exe you might have to use the qualified name.
Or load the acdbmgd assembly from reflection-only context and use Assembly.GetType() .
[CommandMethod("tt")]
public void tt()
{
Editor ed = Application.DocumentManager.MdiActiveDocument.Editor;
Type typeAssemblyQualifiedName = Type.GetType("Autodesk.AutoCAD.DatabaseServices.DBText, Acdbmgd, Version=18.2.0.0, Culture=neutral, PublicKeyToken=null", false, true);
ed.WriteMessage("\nUsing Assembly Qualified Name : " + typeAssemblyQualifiedName.Name);
Assembly ass = Assembly.ReflectionOnlyLoad("acdbmgd");
Type typeFullName = ass.GetType("Autodesk.AutoCAD.DatabaseServices.DBText", false, true);
ed.WriteMessage("\nUsing Assembly : " + typeFullName.Name);
}