I think Georgiana's expressed desire to meet Lizzy came from a letter or letters Darcy wrote from Rosings.

I believe the descriptions (conversations with Darcy) that sealed Georgiana's good opinion of Lizzy might have occurred as late as the ride back to Pemberley from her first meeting with Lizzy.

My point is that Georgiana's feelings about Lizzy need not have occurred all at once. I have presented the scenario I consider most likely.

And although I do not agree with Lesley Mac's idea that Darcy wrote G about Lizzy from Netherfield, I do like toying with the idea, thus:

[Caroline:] "Tell your sister I am delighted to hear of her improvement on the harp; and pray let her know that I am quite in raptures with her beautiful little design for a table, and I think it infinitely superior to Miss Grantley's."

"Will you give me leave to defer your raptures till I write again? At present I have not room to do them justice, as I have just filled up two pages crossed with an elaborate description of Miss Elizabeth Bennet's many attractions and accomplishments." (Ch. 10, additions mine)