Looks like one can bring outside food into the parks:
Guests are allowed to bring outside food and nonalcoholic beverages into the parks for self-consumption, provided they are not in glass containers, do not require heating, reheating, processing or refrigeration and do not have pungent odors. Inform a Security Cast Member of any food items when you enter the park.
Re: sitdown meals around Anaheim - it’s been awhile since I’ve been down there but I remember enjoying Teochew noodle soup from Trieu Chau in Santa Ana and a bowl of pho from Pho 45 in Garden Grove in Little Saigon. I’m guessing Little Saigon may also be a good source of banh mi or maybe some spring rolls or other portable Vietnamese fare for bringing into the park but I haven’t tried any from there.
Here’s a report of some places from back in 2019. Taco María was great for lunch but they have since closed. Also - Burritos La Palma in Santa Ana might be an option for hand-held eats.
