yup, i'm not close to a costco or butcher (dale) and thought i learned my lesson last time, lol. usda boneless beef sirloin choice and all i had was grizzle. reggie had a field day.
The grizzle part I cannot help you with.
The real answer, as you indicate, is to get the best meat possible. These days, that's really hard to do or at minimum very expensive to do, since the premium beef is all snapped up by big business unless you go to a local butcher who will of course charge quite a bit for it. (All IMO/IIUC)
So, you have options to tenderize choice beef. Sirloin is going to be a bit tougher, regardless, but doesn't have to be too bad. You can help it along as much as possible by using sous vide cooking. I do this all the time with steaks from the local markets. I cook them in the water bath (bagged of course) for 4 hours at about 128 degrees, then quickly sear a bare minimum just to put some color on it...many SVers use a torch.
You can pre-marinade it if you want to, as that will help it maintain moisture, and
salt will tenderize if you marinade long enough...but too long and it gets hamy. There's no answer to grizzle, no magic.
And I'm afraid that the way the beef industry is being controlled by the 4 big companies, and the purchasing power of all the steak houses and stuff, we're left with the dregs or paying
through the nose.