What can I do off court to practice?

You can do some powerful stuff, especially against a wall. Try these:

First, without a ball, you could imagine a ball in your hand and go through the motion of shooting and observe yourself. Start with the legs and then, connecting with that power, release the imaginary ball upward toward an imaginary basket. Observe the Release. Is it about 70-75% of full out, quick and strong, yet repeatable? Is your wrist relaxed? Does your hand bounce, being so relaxed? Observe your Follow Through. Is your arm held straight toward the target? Your hand will be relaxed but pointing down. As far as the leg drive energy, notice what percent of that energy you're using? Aim for 90-100%. Catch all of the energy and transfer it to the Release and the shot.

Then, with a ball and shooting against a wall, do the same things as above but now you have a ball to observe, too (height, spin). Working against a wall will be very valuable. The idea is to remain just 4-5 feet from the wall and shoot upward, with the ball glancing the wall on the way down, NOT on the way up or at the top of the arch. This teaches you to shoot UPWARD, and that inspires you to use your legs quickly and powerfully. As you add leg power, just shoot higher and higher, not from moving back, though you could.