The store owner is not caring about the well being of its chicks by selling them too soon. But what is done is done. 30x25x25 is the bare minimum for one cockatiel imo. I have a 60x30x20 cage, which is the bare minimum for 2 cockatiels. If you let them out for most of the day, the cage size isn't a big deal. But sometimes we must leave to work for hours and in that case the bird should have adequate space. I would attempt to slowly switch spinach to kale or another dark leafy green. Spinach is ok twice a week. Her goal weight is 90-110 grams. 67 grams seems pretty small. But now your goal weight is anything above 67 grams. Right now at that age they would normally be 80-90 grams , almost her adult weight. She may have had a slow start in life, or the pet store owner may not have been handfeeding properly. I would keep the scale near her cage, and get her to step onto it for a treat of millet / millet spray. Thats how I weigh my birb. Offer kale, brocolli, carrot, raw or cooked every day. Offer green beans, kale, brocolli, carrots, califlower cooked in a vege mash if you like with some cooked quinoa, this makes it easier when there is no raw food on hand. You can cook a big batch and freeze the extras in daily portions for up to 3 months. Unthaw one portion overnight in the fridge, microwave for 1-3 seconds (depends on microwave) check for hotspots then serve. You can also dehydrate vegetables like kale carrot brocolli sweet peppers in case of times you run out of fresh raw produce. Lastly, you can also sprout organic seeds. Some birds accept pellts that are seed sized, and some accept them in multi colors (zupreem fruit blend for parakeets). Some birds will slowly adjust to pellets if you grind or crush them and sprinkle the dust on their wet vegetables.