It's great she's doing better. You don't need to actually feed her like a baby from a syringe, you can just offer the food on a spoon or from a dish if you want, and you also don't need to be as careful about temperature as long as it's not too hot.
I usually use Hagen Tropican formula as it says it can also be fed to adults who are weak or recovering, but it's mainly designed for baby birds as well, and I don't know if there's really much difference between it and Kaytee. I tried Harrison's formula once since it's organic, but it mixes up kind of gritty and gross looking and Cheeky did not like it. I will also give some treats of nuts and sometimes sneak in a little oatmeal or applesauce or baby food veggie in the formula.
I don't know if this is ideal, but it's the same idea as feeding a pregnant/nursing dog puppy food and it seems to work well. She has laid a clutch of eggs once every summer for 9 years now and is in good health this way.