It has been a MINUTE since I've played an old-school FPS and this really hit the spot. For your first game? You should be incredibly proud, especially considering the amount of time you had to make it. My first game was junk and it took me months to make. Also, no shame in using assets from the asset store. That's why they're there.
You have the foundations of the building blocks of game development. Strike while the iron is hot and keep going. Keep practicing and you'll go far. This game already has a decent amount of things a good game needs. It FEELS good (shooting feedback especially), has an objective, a menu that works, and so on. To that end, you've done an incredible job, first game or otherwise.
Keep it up!