By move 8, hasn't this transposed to a Sicilian?

I think 8. Bxa6 is a mistake. White gives up a bishop for a knight when the knight was not particularly well developed. It would be better to force black to use a few tempo to make the knight useful. White should have been making plans to push f5 and break up the black kingside.

Loomis wrote:

By move 8, hasn't this transposed to a Sicilian?

No.  They look similar, but my database says it's still the Pirc Austrian.

I agree with Loomis that Bxa6 is suspect.  You hit upon a great idea with exchanging your Queen for 3 minor pieces with the open diagonals.  It frequently leads to the lone Queen being pushed around for the rest of the game.

You seem to know that 18...Nd5 is not really an outpost according to the usual definition.

I wish I had more to say about the game, how white could've done better somewhere other than the opening...

10 h3 is ugly but would have covered the g4 square, but it certainly would have been better than Qe1.

10 Kh1 is also worth considering to clear the king from the dangerous diagonal.