in Texas restaurants mostly use the following marinade:
3 parts pineapple juice, 2 parts soy sauce, 1 part water.
If you go to Pappasitos or El Tiempo, this is what they marinade their fajitas in. Also, Pappasitos uses sirloin flap meat. I have seen it at Costco a couple times. Looks like skirt steak and is thicker. But good quality skirt steak with silver skin, membrane and fat trimmed would work just fine. Marinade from 30 minutes to 2 hours. Do not marinade overnight. No need. Cook over a very hot fire about a minute and a half per side. Rest for 5 minutes and cut against the grain. Simple.
For me, I just trim and apply course sea salt. Grill as stated above. Baste with some salted butter when it comes off of the grill and a squeeze of fresh lime juice then let rest.
No reason to overthink things.
