When I was prepping for the sandbag toss at World’s last year, I practiced with a 33 lbs sandbag from Rogue, which I filled with crumb rubber. That was sufficiently dense for the bag to be pretty packed in. The heaviest bag I had to throw in contest was 40 lbs, over 14 feet. So the 33 was a great weight to practice with.
The way I trained it was mostly performing tosses as high as possible, trying to get my direction right, for a bunch of reps at the end of deadlift sessions as an accessory to that. Frequent practice helped me a ton. Try to be as explosive as possible on all training reps.
Your plan isn’t bad, but if at all possible, I’d really try to buy your own rogue throwing bag for practice. It’s really a great piece of equipment to have.
Agree with vinny about not training the lift fresh.
And agree that technique/direction is absolutely the key. Which is why your plan falls short. There’s a huge difference between throwing a bell and throwing a bag, because of the way the handle moves and the bag rotates. The sandbag is attached to its handle by a flexible strap, whereas the kettlebell is rigid. Totally changes the feel and trajectory of the throw. If you want to do well, you’ll have to practice with a sandbag.