Some food for thought: In a dressage test, of course they judge the whole impression. Presentation, the movement, rider aids, the whole nine yards. However, we have the liberty to break things down in our private rides. Other than the five or so minutes you’re riding a dressage test, you can go ahead and break things down as much as you need to to work on them.
Which leads me to my point: it sounds like you have multiple issues that you’re trying to work on all at the same time. This could be a situation where you need to break them down more. Work on your canter aids without spurs. Work on your mare’s responsiveness to the leg with spurs. You do not have to do all of these things at once. It may be you spend a few weeks just focusing on one, then follow up with the other. It may look like you deciding that hey, you don’t need to canter on two or three rides a week while you wear spurs, then the other rides will be more of a transition/seat work focus.
(Edit for clarification: Spurs aren’t a necessary component in transitional work. Spurs are not for forward. Whips are for forward, spurs are for lateral give. So spurs don’t really have any place in transitions, at least as I was taught.)
Back to the canter situation though. If your mare has an auditory cue for canter, use it every time you set yourself up for canter, and work on cantering off your seat and not the leg. Ideally of course she will associate a known cue (sound) with the physical cue (seat) and relate it to the appropriate response (canter).
That said, you’ll still have to set her up for it properly. When I say “a trot so nice she wants to canter” I am not referring to just a nice quality of movement. I am referring to the type of trot and positioning of it (generally a circle, for the purpose of this discussion) where it is physically easier for a horse to step into a canter than it is to maintain the degree of effort to keep that trot. The bend needs to be correct (not attained through pulling the inside rein is the big error I tend to see in riders here), the hind end needs to be engaged and in line (most of the thoroughbreds I’ve ridden have a tendency to want to fall to the outside with their hind end and collapse to the inside with their shoulders). The trot must be energetic without being on the forehand, and certainly not flat or quick (again, can be a TB-related concern. They get flat, on the forehand, and rush fairly easily by nature of conformation).
Once you get this trot, you have something you can work with. Make sure your inside rein is light. (So many riders - myself included - have a really nasty habit of wanting to grab at the inside rein. It is not your friend! Imagine that the inside rein belongs to your horse, the OUTSIDE rein is yours.) And then the canter aid. I’m going to take a stab at explaining this but I will inevitably make a mess of it I’m afraid…
Make sure your body is lined properly (and for sake of this description, imagine asking for the transition on a 20m circle). Your outside hip is drawn back a little bit, your inside hip is level (NOT pushed forward). Your upper body should be rotated a little at the waist to bring your inside shoulder back (in line with your inside hip) and your outside shoulder can be a little further forward (by nature of the rotation of your waist - NOT that you are pushing your shoulder forward). This position is really awkward to consciously think about - but it helps you set your body where it needs to be, and keep your weight/balance proper as well. (It’s also something that I found came back to be reinforced with me once I was working on half passes and flying changes, so establishing this as a solid foundation now will probably serve you well in the future.) So with your body position, let your outside leg come back from the hip - it’s not just bending the knee to jam the calf/ankle back. Let your entire thigh shift a little. And then with this position (and the trot you’ve created above!) think of swinging your inside seat bone a little forward. Sometimes it helps your mental imagery if you imagine swinging your inside seatbone to the outside shoulder. You shouldn’t be shoving or scooping your seat at all but the image of “scooping” them into the canter can help some riders, I think. With a light, balanced seat and a giving hand, this should be a decent set-up to go into the canter.
And of course - if it doesn’t work/things fall apart, go back to the basics and build it up again. Do not keep chasing when the quality of the gait or the position has been compromised. A frenetic, rushed canter transition doesn’t school/teach/reinforce anything you want. So don’t do it.