Firstly wondering why you feel the need to feed a good doer that's not in work any grain at all & why you want to experiment??
Starchy, grainy feeds aren't generally the best for horses. It's important that if feeding, they're fed little & often & any dietary changes are made very gradually over a couple of weeks.
Of those feeds you listed, the links provided no info unfortunately. Don't know anything about the Dumor. Nutrena & Purina don't provide ingredients or much info on their site
, but I think ingredients of SC & Strategy both include corn & wheat in both cases, which I'd not feed. Nutrena Safe Choice is much lower NSC than other Nutrena feeds & lower than Strategy, but apparently still 26% which is high. Alfalfa is a generally pretty good feed for horses, *as part of a balanced diet* but is high energy, so should be judiciously fed to 'good doers'. I'd opt for hay/cubes over pellets, as for one, there's long fibre, & more chew time which is very important for horses & for 2, you don't know what else is in the pellets - dust eg, as it's often made from floor sweepings.