One parent in mid 80s and one mid 70s.
Breakfast - porridge microwaved with honey, milk, fruit - banana usually
Lunch - toast with tomato & cheese, jam, tuna or cream cheese spread
Dinner - two or three independent meals per week, breaded fish/fish fingers in oven with boiled baby potatoes and peas, mini chicken fillets in a jar of curry sauce with rice, fry or omlettes
Older parent cooks and it is becoming difficult for them to stand, they ask the younger parent to help but their focus and ability is impaired so need to be supervised so things don't burn or are taken out raw. They don't like ready meals but will sometimes have a prepared lasagna or pie.
I provide the other dinners each week and sometimes soup for lunch and some baked goods. It's hard to watch them struggle preparing meals but I was told not to take their remaining skills from them. The shopping is done for them and they always have lots of fruit, biscuits, bread, yogurts, crackers and baked goods to choose from.
Some medicines reduce appetite and some increase it, it can be difficult to balance.
If the ready meals are not being eaten you could try and ensure there are some healthy snack type foods.
My parents would refuse food initially, they don't particularly love my cooking, but I always insisted it would go to waste and they don't like waste. It's taken a while to get to the level of acceptance we're at and it can be overwhelming to be responsible for everyone so see what you can manage easily.