I’m a 14 YO pitcher. I just recently finished playing summer ball. I’m about to start a weight lifting program to prepare for school ball come next year. I’ve topped at 61 mph and even though that’s below average for my age I’ve always been one of the better pitchers in my league due to the depth of my arsenal and plus command. I throw a fastball(4 and 2 seam) and a slider. I can control both pitches well. I also throw a curve that I can throw in any count and a changeup I plan to develop over the next 4-5 months. I know this is a wide arsenal for a 14 year old but I throw everything like a fastball. I don’t pronate or supinate on any of my pitches aside from the natural pronation you get after pitch release. I also am going to work on my mechanics to build more powerful, safe mechanics to become a better pitcher come time for the varsity season. Anything else I might need to work on besides becoming stronger and faster and gaining velocity?
That’s plenty to work on. You don’t want to spread yourself to thin and only make marginal improvements at best in each of those areas.