Honestly the thai food in tokyo is incredible but tourists always end up at the wrong places. Real thai restaurants cluster in areas with thai workers - takadanobaba near waseda university has legit som tam that tastes like home.

Try little bangkok in shin-okubo, they do proper isaan food and the som tam actually has the right fish sauce funk. Prices are reasonable too, around ¥800-1200 per dish. Avoid any thai place in shibuya or harajuku - they're all adapted for japanese tastes and taste nothing like real thai food.