Is the teen with ARFID able to prepare their own food? That would mean a little less for you to do. If they eat beige and you don't already have one, an air fryer will be good for this as less likely to burn/set on fire if neglected.
Is the other teen able to cook for you and them once or twice a week? Even if it's just a jar sauce pasta dish? Heinz sauces are non UPF, taste OK and are often on offer.
What do you all have for breakfast and lunches? If you work in an office, can you take leftovers that might otherwise be uneaten for lunch?
Can you agree a list of dishes you and non ARFID teen will eat on rotation? Is there anything that the ARFID teen eats that you can all eat together?
What about simple meals sometimes, omelettes, baked potatoes? Either routinely, or as the alternative if teen doesn't want what you've made.
Also do things like make a basic ragu that can then be turned into bolognese, chilli, lasagne? Or chilli where the leftovers then get used in enchiladas by baking in wraps? Likewise leftover roast into a pie (with pastry or mash topping as appropriate), curry or with egg and chips etc?
I would buy as little as possible short life fresh food where you know it might get wasted and concentrate on keeping your freezer stocked with healthy(ish) foods that can quickly and easily be turned into something edible.
ETA - yes, a slow cooker could be another help. But what will help depends on what sorts of foods they will eat and how much they're able to do for themselves.