SweatLab

You won the minimum. This reg 3-bets light and calls down with any pair, so against him AK is a value hand on every street: 4-bet preflop, bet the turn when he checks, and bet 30bb on the river, not 12. The lines you took are the balanced ones; against this player they make about half of what the aggressive lines make. Nothing in his line says trap, so nothing here called for caution.

1. Preflop4-bet to 25bb. He 3-bets light from the blinds and calls 4-bets with the same hands he 3-bet, so AKs is a value 4-bet against him: he continues with KQ, KJ, AJ, suited broadways and small pairs, and you are ahead of all of it. Flatting keeps his bluffs in, but it keeps the pot small against a player who will pay you off. The read this depends on: that he calls 4-bets rather than folding them. Watch what he shows down after calling one.
2. FlopCall is fine; a raise is better. His 7bb into 20bb is the bet he makes with his whole range. Against a player who calls down with any pair, a raise to 22bb gets called by every K-x, 7-x and pocket pair, and you beat all of them. Raise.
3. TurnBet. Checking back top pair, top kicker against a player who calls down with any pair is the biggest leak in the hand. His turn check says he has a pair he wants to take to showdown, not a hand that will bluff you. Bet 25bb into 34.5bb; he calls with K-x, 9-x, 7-x and pocket pairs. On this read the check cost you about 25bb.
4. River12bb into 34.5bb is a size for a player who might fold. This one calls down with any pair, so size for his calling range: 30bb gets called by exactly the same hands as 12bb. 'Rarely bluffs rivers' is a reason not to call his big bets; it is not a reason to bet small into him. His check tells you nothing new, and nothing in his line says trap.