I have seen a lot of turn-based tactics games where enemy units simply stay idle unless there is an enemy in range. This isn't necessarily the strongest solution and might allow a clever player to cheese if they can somehow find a way to damage enemies while still ending the turn outside of their movement range (Some of the Disgaea games allowed that exploit under some circumstances). But it can still be a "good enough" solution for many games. It also allows you to create longer combat scenarios where the player army slowly progresses through a larger but sparsely formatted enemy army step-by-step. If the AI is too aggressive at moving in on the player instead of waiting for them to arrive, then this becomes very difficult to design.
But let's say you do want to go the extra mile and have an AI which behaves more aggressively.
Just walking in the direction of the closest enemy unit is a valid option. I would prototype this, playtest a bit and see how it works out. But if you want the AI to be extra smart, you could use the rating function you are already using to determine targets, but apply it to all enemy units and add the distance as another factor. So the AI will balance target distance with target desirability when deciding which target to close in on. The ideal balance factor is a matter of preference. A "perfect" AI would measure distance in turns, not in tiles. But measuring by tiles might make the AI appear more human. It would replicate the human bias for attacking enemies which look closer even though from a purely game-mechanical view all units which can be reached within one turn are technically the same distance.