Hiya Dominique, I apologize about the delayed response. I'm proud of you for taking the beginners riders course, they really give you some good foundation skills to build on. I've answered posts similar to this before and I'm gonna tell you the same thing I tell my boy ( he's 15 and gonna be a licensed driver sooner than I care to think about lol) He wants a suped up race car sooner before later and I'd prefer he learn in an 86 Honda Civic handed down by somebody's grandma, you know the type with rose stickers in the back window? You could make a small investment in an older but reliable inexepensive learning bike, then you won't have to be sad when you take on scuffs and bruises, your Harley will happily await your progress safe in the garage.
I wouldn't waste time with a 250 because they are hard to sell when you're done with them, but perhaps an older Honda Nighthawk or Suzuki intruder, something like a 650 cruiser won't hurt you on your insurance and will deliver enough power to get you from point a to point b. I'm a firm believer that parking lot practice at slow speeds is crucial to safe riding for every rider, expert or novice. Get you some sidewalk stick, some rubber duckies and some bottled water and find a lonely church parking lot on a weekday evening. Set you up a slow riding course and practice starting and stopping, practice figure 8's and swerves, quick stops etc just like you did during your riders course use the rubber duckies like road cones and you'll hear them squeek if you hit them. Get ya some level headed riding friends who just wanna go out to have fun who won't encourage you to ride faster than the speed limit. I promise start and stopping practice will remedy the stalling issues. You're only a bit shorter than I, so your height is an advantage. Make sure you have some good comfy riding boots with an acceptable heel , get a thicker sole if you need the height but make sure the extra width doesn't make too tight of a space between your peg and your shifting lever. Once you have a couple thousand or so miles under your belt, I recommend taking an advanced riders course with your own Harley. Please keep us updated with your progress. Have fun with your adventure and ride safe.