The most important elements in Casting a longer Spey line a long distance are anchor placement, a taught, powerful V loop, with no slack, and it doesn’t hurt to have a long, powerful forward stroke.
I think a great way to practice is to work mostly on the V loop and anchor placement before worrying about launching too much running line. Also, I can get more reps in shooting a moderate or less amount of line while concentrating on keeping slack to an absolute minimum and accelerating the last third of the sweep. The rest is timing, which mostly takes practice. Hours and hours of practice.
Breaking down the single spey cast a little bit so it all doesn’t sound so mind-boggling. It might be helpful if you’re just starting out.