One way to look at the situation is this:
Without your care & intervention, Tia would probably not be alive. The fact that she has shown such remarkable healing is a testement to your care for her & the (frustrating for both of you) confinement in the smaller cage.
I'd use that as a baseline & anything Tia gets above & beyond that is a bonus - so every bit of extra time she gets to potter about gently as exercise = a bonus. Every time she gets excited to see you & you develop a deeper bond = a bonus. Every time she gets dust rubbed into her fur = a bonus ................. that way you're not looking at what Tia has "lost" compared to other chins but you're looking at the amazing life she has gained because of you.
I think Tia is a very, very lucky chinchilla - some chins live in dreadful conditions or in teeny cages with no real human contact - Tia has landed right on her little feet with you.
Slowly, slowly is the way to go, as Peggy has suggested.