I think you are doing a great job!
. Squirrels are finicky and unpredictable with what they choose to eat. But think about all the different stuff they eat in the wild. One day they may eat a flower, then not touch one for a week. I think it's the same when they're captive.
A couple of weeks ago I listened to a talk about squirrel nutrition given by Dr. Emerson, who is one of the best squirrel vets around. Based on the squirrels she has seen in her practice, she feels very strongly that a captive squirrels diet should be 75% rodent block, like Zupreem, Harlan Teklad, Mazuri or Oxbow. The remaining 25% be veggies, with just 1-2 pieces of fruit a day. She also feels that they should be offered protein in the form of worms, bugs, eggs, etc. These are the things they would eat in the wild.
As for sun, she recommends 30-60 minutes a day. A full spectrum UV light can be substituted. The bulbs should be replaced every 6 months because although they will still work, they do not emit enough UV rays after that time.