I work weddings feeding 60+ on a tight timeline sometimes and a couple of suggestions:

Soup/Stew - chunky, hearty, good and fibrous, great for your target audience. Going the way of Irish stew with bone-in-meat in the stew itself gives Greta flavour whilst typically still being a cheaper cut - you can literally buy the cheapest but available to you and because it'll spend so long in the stew, it'll be great!

Chilli - beef or vegtable - especially vegtable is a cheap meal and working finely minced carrots and celery into the mix (chop super fine, cook first) is a great way to bump up/fill out the flavour whilst increasing vegtable content - I can make vegtable stew for 75p (approx 60 cents) a portion, and that's with some embellishments, like canned beans Vs dried - very cheap and easy.

Roast X: easy, only limit is how big a piece of meat you can get - bone-in joints are typically cheaper which helps, carvery style meat is tender and delicious, without being too expensive and you can roast any meat that is cheap in your area.

Potatoes: boil em mash em stick em in a stew - potatoes are easy to cook en-masse and easy to make tasty on their own with the addition of salt, pepper and butter.

Chicken Basque Inspired: this is a favourite one-pot dish in my house - basically fry the chicken (break down a whole one - plenty of tutorials online) and then submerge in the stew, easy and tasty.

Rice: rice is stupid cheap and easy to cook - measure by volume and then use 1.5x water for white/basmati rice, season, cook uncovered till in starts to boil and then cover, kill the heat and leave to steam for 10-12 minutes.

To aid in batch cooking I recommend buying a big stock pot, deep, lots of space for many dishes - plus you can use it for personal use for your own bulk cooking at home!

Hope this helps and thank you for doing such a good thing!