Value Extraction & Line Check - AK in position on KKTr flop

1/3 - effective stacks $350ish

Villain is 30s, Asian, recently sat down at the table and has only played 2-3 hands in last 15 min, but didn't showdown anything. No real reads.

To the hand:

UTG (Villain) straddles to $6, action folds to Hero with AKo on the button, hero raises to $18, both blinds fold, UTG calls $18

($36) Flop KKTr

UTG checks, Hero bets $15, UTG calls

($66) Turn 5s bringing backdoor flush draw on the board - Hero does not have a spade

UTG checks, Hero bets $40, UTG calls

($146) River 9c

UTG pauses for 10-15 seconds then checks, Hero pauses for 10-15 seconds and bets $20, UTG calls

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Did we miss out on any value here? What sizing do you like across all 3-streets?