I'm going to be giving quite a bit of harsh feedback, but please understand I'm trying to present ideas so that you can possibly take this idea and make a really good game out of it!

The control scheme could definitely be taken somewhere, but without any player feedback or challenges the player needs to face, the game becomes really boring to be completely honest. Furthermore, it really isn't made clear what the player is supposed to do. Sure, it's possible to figure out, but a simple screen just explaining to the player what they need to do can go a long way to making the game more polished. Finally, the setting really doesn't fit the theme, so it feels less like an interactive game and more like a chess board.

So, here are a few ideas that I personally might implement to turn this idea into a fun game:

1. You seriously need to add a challenge. The best one that I could think of would be to include an AI opponent, so that the player would need to be on top of their game to make sure that their points aren't taken away from them. Or, even better, allow multiple people to compete for points at the same time (Not sure how you'd pull it off, but it's certainly worth a try!). Even adding a game-timer will pressure the player into getting more points faster, and adding a high-score system will keep players coming back.
2. Increase the pace of the game. Make the animations much quicker, and once again an AI would also work great to increase the pace of the game. The faster the pace, the more challenging it will be for the player to master the game. Adding a pace that gets quicker as the game goes on will also create a very natural difficulty curve. If you see the game that I created, you might notice that the zombies will actually get faster as the game continues. 
3. Add feedback. By this, I mean add things like sound effects and visuals to let the player know exactly what's happening. Here are some musts for sound effects:
   1. A sound effect that plays when a player selects a direction
   2. Another sound effect that plays when a player deselects a direction
   3. A sound effect to play when the player makes it to the target
   4. A sound effect to play whenever a move is completed by the tank
      
      Just adding these few sound effects will increase the quality of your game significantly!
4. Download some free-to-use models and decorate the scene. Epic Games have a MASSIVE selection to choose from, and creating a scene in the background will do an amazing job at increasing the quality of the game!

Once again, I know it seems harsh, but I genuinely believe this control scheme could work really well in creating an interesting game and I'm just trying to help. If you want to do more research into making games more fun, Jonas Tyroller has a massive collection of videos about it on his YouTube channel (the channel is just called Jonas Tyroller).

I hope this helped!

so cool how you combined the card game mechanics with the grid based-movement!

as others stated, it could be a bit more challenging for example with a limit to the cards, certain cards that must be played or some other objectives on the stage, this has a lot of potential!