I started to pitch when I was 10 and kept it up through high school but after junior year my coach tried to convert me to side arm. In the end I taught myself to pitch sidearm. For velocity, I highly recommend the Driveline Baseball throwing program. As for command, I had to teach myself command because I never had good command growing up and found out fast that if I throw side arm it was very easy for me to hit a lot of people. To teach myself command I watched a video with Pedro Martinez talking about how he learned command becoming a starter (I’ll find the link and comment it). As for pitches I already commented my thoughts on another one of your posts so check it out and if you have any other questions let me know!
P.S. you say that being side arm limits you too the bullpen. Gladly I can say that is wrong. You can be an ace of the rotation, anyone after him, first guy out of the bullpen, a nasty closer, or anything in between. Shoot for the stars!