Another way to visually recognize when exactly you can slidecast is to assign any emote into your hotbar's free slot then watch it as you spam your GCD spellcast.

When you start casting, it will grey out like any other icon in the hotbar. But for some reason that emote will light up first before anything else as you're about to finish your spellcast. THAT is exactly the time you can start moving your character to slidecast successfully. Not something I'd recommend because it forces you to eye your hotbar but it might help to just know that. You can take it off once it becomes second nature to you.