Establish a baseline for your command right now. Chart the variance from your target on each pitch. Throw a bullpen in blocks of 10 identical pitches and calculate average miss. Track it over the off season and see what kind of results that focused effort got you. Throwing the pen in blocks of 10 identical will allow you to groove your release point for specific pitches, then recall that feeling to execute the pitch in a game.
Shotgun bullpens where you mix it up constantly do not help overall command or accuracy, they only help maintain or develop arm strength. Incorporating focused blocks of pitches accomplishes both.
Make one bullpen all focused on the inner 1/3. The next one can focus on outer 1/3, bottom 1/3, upper 1/3, etc. One pen could focus down and in, next down and away, then up and in, then up and away. Structure it how you want, just keep track of the results.
Velocity, comes from perfecting your specific mechanics and conditioning your body to be balanced and strong. It’s much more difficult to add significant velocity, so that plan needs to be more detailed and you must stick with what gives you the best results.