I agree with you that Darcy is an introvert to Bingley's easy sociability and extroverted temperament, which would at least in part make his more reserved behavior at the Meryton assembly understandable. But anxiety is not a universal characteristic of introverted types and I don’t think that we have any evidence of Mr. Darcy being anxious. Darcy is described as being “haughty, reserved, and fastidious” (ch 4), but not anxious. He is also described as “clever”, so we could surmise that he is smart enough to be aware that news of his wealth would be a topic of gossip in a setting like the Meryton assembly, making him the object of interest to the Mrs. Bennetts of that part of the country, who would be falling all over each other to marry off their daughters to him. I could imagine him as choosing to be aloof at the Meryton Assembly in order to keep that sort of obnoxious and unwelcome behavior to a minimum. On the other hand, his unflattering comments about Elizabeth, made so that she could hear him, were, IMO, inconsistent with any sort of "well-bred" manners that Mr. Darcy is described as having (ch 4) and he just comes across as rude.