Encountered this a few days ago. AFAIK there's no way to spend Villagers freely on command. If this is true, this becomes a problem as when Flourishing Trade is active, you may want to spend villagers to spend actions as buys. When you are all out of actions and buys, your turn ends. Based on other interactions in the client I assumed that clicking at what you'd like to buy would trigger spending a Villager as a buy when you have no actions or buys, so that would be my proposed solution.
Thank you for your continued work on the client.
Yes, it is indeed a problem that the game doesn't stop to allow you to spend Villagers when your Action phase ends.