You can have productive rides without ever broaching the canter issue. I advise you consider the possibility that if you want to continue wearing/working in spurs, then perhaps you don’t canter on those days until you sort out your transitions. A horse doesn’t need to canter to be working hard, have a productive ride, or be learning things. Not cantering a few days out of every week’s set of rides will hurt no one (and in fact, as you work on your transitions, will likely benefit as you won’t run the risk of accidentally catching her).
Ultimately, it seems like you have too many moving parts happening. The spurs question, your equitation, her responsiveness to aids - there are so many things that it is going to be hard to work on every single thing every ride, every moment you’re riding. Because there’s the thing: when you are wearing spurs, if you make a mistake, you are punishing your horse. You do not mean to, but you are. So then you’re careful to not make mistakes - which inherently is good, but riders often ride less effectively/more defensively/lose proper positioning when they’re struggling with these issues.
If the original question of this thread is “How to cue for the canter when wearing spurs” my response is: “The way you cue for a canter any time.”
However, your cue for the canter is poor - you’ve mentioned you struggle with it. This is completely unrelated to spurs. And with this information, it means that perhaps you shouldn’t be wearing spurs while you work on ironing out your transitions into the canter.
Once those are working better for you and your mare you can revisit the idea of wearing spurs 24/7, but until then I would suggest picking and choosing which days you wear them, and narrow your focus to what you’re working on accordingly.