Sometimes I'd lean down while giving them a neck rub and say to them "hey, we're gonna have a fun canter in a minute but I don't wanna go too fast OK?" And then once cantering I would literally put all my trust in his training, his personality and my instructor and just enjoy it. When it came to coming back down I'd also say "hey, we're gonna stop now, please" sitting deep, and slowly asking instead of hanging onto his mouth or jabbing him sharply.
I'm curious now what kind of dialog others have with their horses, knowing full well they only recognize the emotional tension in your voice as cue, not what is actually being said.
For canter, with a visibly excited horse, waiting for my cue: "All right, go nuts!", with the most delicate of calf squeezes.
For down-transition: "Aaaaaaaaand trohhhht...trohhhht...", then "Aaaaaaand waaaalk...waaaaalk", accompanied by increasing inertia in my body and some wiggle action in the reins.
People only ever praise their horse after they are back to walking ("Thank you for not killing me!"). I started telling horses "Good boy/girl!" after a good up-transition, and if I can swing it, I even try scratch their neck. (Of course if you are posting, a gentle scratch at the withers is often the only feasibly reward.) In line of what you said, I also believe that a nice calm "Good boy!" right after the canter transition sets the tone for a nice calm canter.
Two-point, for the entire time or just as temporary relief, really helps with extraordinary bounciness at increased speeds or to collect yourself back into the rhythm.