Nice simple 2D top down shooter. With top down shooter games designing AI logic is difficult because first you need to settle on player control mechanics. Based on player mechanic and the limitations that are presented to the player will depend on how complex of an AI will be needed to designed for the game.
In your case you attempted to do all in one go which for starter project is good as you gain experience on how to make a game from start to finish but difficult for a beginners project. As in a 2D top down shooter genre there will keep appearing game aspects that may be needed to be made to improve the overall gameplay.
A list of game aspects are player movement, weapon choice/selection, AI design and there corresponding problems such as when does the player movement change and by how much, weapon choice how to store memory and where with their respective characteristics, and AI difficultly and possible decision making.
Overall well made from start to finish just need more practice. Create smaller projects with either unique or already existing game mechanics altered to learn about game design in more depth.