Agree with boramiNYC. Years ago I had a friend to went to small junior college and over 1/2 the tennis team was from overseas on a scholarship. My friend was good - very solid 4.0 player who worked in the summer as an assistance coach in clinics. He could not make the team. I have played other guys who played D3 and were not that strong. So, there is a big variance in ability.
If you really want to improve, get some lessons.
1. Your slice BH needs re-engineering. You lead with elbow and don't use the shoulders much.
2. I only saw 1 volley (didn't watch all the video) and it was not good. Your hand should not move much on a volley. Your hand moved about 3 or 4 feet. I am not going to try to write how to volley - get a few lessons.
I know lessons are expensive but if you really want to play D3, get some lessons and continue to work hard. Maybe you and your sister could share a lesson to reduce costs or maybe you have another friend interested in lessons. You may also want to look into summer camps 1 week long or more. If they are in your city, you might be able to stay at home. A camp usually is 3 hours or more per day of group drills. Not a lot of individual instruction but you will hit thousands of balls and get a few tips on obvious weakness.