Well normally I won't be critical about game design and focusing on the ideas. But the game got potential to be great so listen to my advice here.

Game design is a good start if you want to improve your overall game development.

Let me point out the most obvious fact which ther others pointed out. The game loop is bad.

Games like this should be focusing on the short loop. For example, limit the scores instead of waiting until the 5 minutes ran out to count the scores.

It's better to think similarly to how sports work when designing games like this.

Like volley ball or tennis, you'd generally have a limited sets or scores to play in order to win.

Let's look at Rocket League, you kick the ball into the goal and gained score. The loop for each "game" is just until you scored the goal. Then the table reset. Now everyone got the chance to start again. Until the "match" ended, which generally be whomever got the predetermined scores first. Maybe some special rules like when the scores were too close, the match will continued until the hard line instead of ended right there and maybe have the final score to fight for if the match was super close.

I know that you might try to break out the conventional stuffs. But before then let's focus on the foundation first. The AI will always be more efficient in movements than players, though they might not be as smart if you don't make specific rules of how to get more score.

Maybe start with the arena smaller, the ball smaller, and slower movement to see what kind of rules you want to use. Then expand from there.

turn down the audio 

If you shortened the rounds and added variables to switch up how you play, like walls that block your shots sometimes, that would make this a really good game. You've got serious potential here.