Until you get hired, don't worry about a duty gun. They will either give you one or tell you what you can carry.
Don't worry about getting a leg up for firearms training, it's usually easier to teach someone that doesn't know anything rather than someone who already "knows" what to do. Saves a lot of time untraining them. But, if you already enjoy shooting, have at it. It will neither help you, nor hurt you in the hiring process, unless you come off as a gun nut. (I am, I hid it well)
Remember, it's not practice that makes perfect. It's perfect practice that makes perfect.
Your Beretta is honestly one of the most tested and used guns in US military history. It's a perfectly good gun. It doesn't fit my hands very well, but, that's my problem. If I had to carry one, I would be happy.
Slow down, don't sweat the little crap right now and, get hired. Been there, done that for almost 30 years.