This is not qualified dietician advice but just one of common sense and a basic knowledge of food types.You don't have to eat protein at every meal to get muscle gain. It is the regular use of a muscle that creates muscle.
You sound to be lucky to have a slim body type, and it is really beneficial that you are taking an interest in how you provide your body with nutrition. Work at good nutrition and your body will be naturally fit and healthy. Work at your gym workouts and other cardio work and you will get the muscle definitions you aim for. Just enjoy what you do and don't fuss or obsess. You cannot really change the body mass type you were born with, that is genetics for you.
If you are not trying to lose weight it makes more sense to ignore the protein emphasis and look at your fresh fruit and veg food intake overall. Your daily food intake should be a balance between proteins, carbohydrates and fats to give your body system the calorific intake it needs.Your pre formed protein bars are shockingly expensive but have been marketed for the 'younger market' as healthy (these are all processed foods) Your food intake should avoid the many non processed foods, pre-formed, shaped foods as possible (ditch anything with a a lot list of e numbers, coatings and chemicals)
Try to eat fruit and veg that has been harvested without processing into extruded, reformed shapes, avoid fries, chips, ready meals. Avoid cakes, biscuits and chocolate bars (high quality dark chocolate the exception) Start looking at introducing 'whole food' items into your diet. Your diet is so crucial. The numbers of children and adults suffering from constipation as a regular experience in the Western world is shocking, it is so avoidable. Your poo should be soft and easily expelled. If you bulk up your diet with natural fruit and veg fibre your gut will thank you and so will your your long term health. Your gut health is as important as your physical health and there is evidence that a healthy gut can help you avoid disease and infections.
This assumes you have no specific dietary needs or allergies etc
East porridge oats and fruits as breakfast, multi item fresh salads (baked potato, boiled new potato, sweetcorn, green leaves, tomato, cucumber, olives, grated carrot, beetroot, blanched green beans, courgettes) dressed with olive oil. You talk about the costs of eating chicken but on a cost for cost basis raw chicken is as cheap as protein mix. If you look at trays of chicken legs, breast, thighs, or other meats and freeze your individual portions. Don't forget fish, nuts, meats, cheese, milk, yoghurts, pulses as protein sources. Have a look at food price reductions in supermarkets refrigerator isles and change your planned meals against what is reduced for the day.Use an air fryer to cook meats quickly and cheaply, or simmer them with vegetables for nutritious soups or stews. Drinking skimmed milk is a very good muscle recovery and protein item. So are the many dairy products, natural yoghurt, cheeses etc.
The Leon takeaway group do very good books on fast suppers using whole foods and their recipies are very good, but there are many more all extolling the virtues of fresh whole produce. Drink plenty of water, and add a squeeze of lemon or fruit juice juice for vitamin c.
Hope this helps?