I implemented expectiminimax in the browser which allows for a fairly strong AI player: https://keshav.is/coding/pt3/

I found that once you naively search the game tree beyond a depth of 8, it more or less converges on a particular choice. The presence of a neutral outcome (i.e. neither player claims the selected square) means the tree depth is technically infinite, but feasible to search thoroughly once the first few moves have been played.