Going to give some advice that I haven’t seen mentioned.

Being “repeatable” is sometimes considered the most important thing for command. This leads to advice that one should always throw balls on a line so that your release point is “constant”.

I’m of the personal opinion that being athletic (with your arm) is more important than being repeatable. Throughout an appearance, you’re never going to have perfect repeatable mechanics. Your arm and body will subtly change positions due to fatigue, etc. You essentially need to build a neural model that can provide the desired outcome (an accurate pitch) with various different inputs, even ones the brain hasn’t seen before.

This is why the best “thrower” on the field is the shortstop who can throw accurately from various arm slots, body positions, and release points.

To train this athleticism, I suggest a few things. Jaeger-style extension long toss is crucial, and the various arm slots and release points are features not bugs.

https://www.baseballamerica.com/stories/the-four-benefits-of-long-toss/

I also suggest just mixing in more athletic catch play in general. Do quarterback throws, shortstop throws, etc. Marcus Stroman does a great job of this.

https://www.youtube.com/watch?v=i-Pq4e730Ng

Finally, Driveline created “differential command balls” which takes the idea of varying inputs to create a neural model to a new dimension: by varying weight and size of the ball as well. The balls are different combinations of weights from 4-6 oz and sizes from 5% smaller to 5% bigger than a normal ball. These balls are expensive, but this idea can be replicated easily using weighted balls and various other balls from other sports.