Why not try some of the following:
1. Stop bots trading with the leading player if the leading player only needs 1 or 2 VP to win. [DONE]
2. Stop bots trading 2 cards for 1 unless they are already over the card limit by 2. [DONE]
3. Start mining all the saved games and build a decision tree based on winning human players' actions. Different decision trees are needed for each phase of the game.
4. Run simulations with just bots testing out the different decision trees and promote the wining combinations to be used against real players. It should be possible to run thousands of hours worth of simulations in a fraction of the time.
5. Invite human winners to play test "best of the bots" and offer feedback.
6. Give bots personality slants (e.g. "city boy", "merchant", "explorer") so that players can choose what sort of bot competitors they want to play against.