JustCleo
I had a 10 yr old DD with ADHD and Aspergers (now called ASD).
We have also had (and continue to have) massive food issues - she will eat certain things, go off and back on others, constantly refuse some, etc.
DD was always slight, and always had a smaller appetite. And now that she's on meds for the ADHD part, they also affect her appetite.
But I have learned to step back.
Firstly - concentrate on making sure that your DD is eating - SOMETHING!! She looks like she does eat a variety of foods - congratulations. Make sure she has access to enough actual calories however they are made up. And fats are important, so while "crisps and cake" sounds stupid, they contain fat which is essential for growing bodies and running around and developing brains/nervous systems as well.
Does she drink milk? Keep that up and make sure it's full fat for her (we have low fat for DH at home and full fat for DD - I now only use skim at work to avoid overcrowding fridge!). And my own DD was still getting a bottle/large cup of warm milk at bedtime way past "give up the bottle" time - she still takes a smoothie-maker bottle (likes the big opening) of warm milk or a large mug of hot chocolate most evenings at bedtime. Not only is it useful from the "warm milk to wind down" at bedtime perspective, but its extra calories to fill her belly and get absorbed overnight so she's not hungry. And if it's hot chocolate, I add squirty cream (from a can) and marshmallows - she thinks its a treat but I see the need for the fats of the cream and the marshmallows are an incentive to drink it all.
Cheese is great - especially if it can be a variety of cheeses. Cream cheese added to sauces, or grated hard cheese on top of toasties etc or in sambos. And, if she is open to trying new flavours of cheese, it might be a way to slowly widen her tastebuds in time.
DD won't eat a sandwich for school - so has chicken nuggets in a hot food flask most days instead. She will now look for the bag of popcorn and cereal bar in her schoolbag if she is still hungry (for a long time, there were raisins or cereal bar or popcorn that were ignored most of the term - but she's finally eating them on hungry days) - I leave things that are long-life in there, and ignore them once I know DD knows they are there (just check if need to replace).
I also know that she usually comes out of afterschool club in foul form - partly overtired and verging on meltdown from coping, but often something like chocolate immediately in the car will help. She nearly always wants something (but will now accept cheese, cooked meat or tomatoes instead of "junk") as soon as we get in before dinner is ready. But that was a slow move to "healthy" stuff, and there are days when a packet of crisps is needed then.
Also, and I know it is not always the case, but once DD started to cook herself, she would eat what she made. So some evenings she will make her own snack (maybe cooked chicken in mayonnaise mixed with mild curry powder) while I cook dinner, and she can also make a few recipes herself entirely so if she really won't eat what I've cooked, I let her make penne arrabiata for herself (the most frequent request) - pasta, tomato sauce (from onion, garlic and passatta, and chilli powder), bacon pieces. As she often only objects as I am dishing out - so I can use her portion for my own lunch tomorrow but I am not starting on making another dinner myself at that stage. Tuna melt mix is another she does herself - and that is easier for smaller ones to learn too - we make it with tuna, sweetcorn, spring onion, red peppers, mayonnaise and sometimes tomato as well - and if they get involved in making a bowl for themselves and having control over what goes in (Mum and Dad like peppers, but you don't so yours won't have them) and the amount (you like more sweetcorn than we do, so put more in yours). We do grow some veggies in the garden too - DD always loved peas straight off the vine, and tomatoes too. And I always keep peas in the freezer (petits pois rather than larger peas - it matters!) in case she'll eat the dinner but not the veg, or just have plain pasta and peas sometimes. Sauce separated from rice, sauce in the middle of rice, no rice, no sauce, sauce but no XY or Z in it.....we've been through them all!
Yes, it feels wasteful, and like you are not doing right by "giving in" - but you're not. You are getting nutrition into her. And eventually she will (hopefully) start to get better habits and wider variety. And there will be "2 steps forward, 1 step back" times (or even 2 forward and 3 back again sometimes!). But honestly, when I started to let go the reins on "you need to eat healthily and properly", it made a lot less stressful household.