Good job! I start mine in soil now and have three, 1 year old, 16 - 20 inch tall trees that I just brought inside for the winter. The hardest part for me is keeping them inside over winter and maintaining the watering schedule correctly. I have found over the years of trying to keep avocado trees alive that if I can get past the first year, the tree is generally strong enough to keep going.
I lost my best prior tree to forgetfulness and a sudden cold snap. Still annoyed by that to this day.
That's a real shame! It's really difficult to monitor watering once you've transplanted to soil. I lost my first attempt after transplant shock was too much for the plant. I hope you continue with a good over wintering. Good luck :)