Thank you for your question!
In my opinion, there is almost never a moment when you can say "Ok, I move to the next step".
I see the evolution of my horses more as a constant "work in progress". It means I constantly try new things when I feel my horse is mentally and physically relaxed. And that I know that the first trials will not be very good and even sometimes quite messy!
But after every trial, it gives me more clues about what to work on, what should be improved, and I keep moving in the direction I want to go.
To be a little bit more specific about your question, there a still some "check points" that I want the horse to integrate before I try something new.
For example, I will not try the travers and half-passes before having the feeling that my horse understands well the response he has to give to my inside leg.
The important thing to remember is that the rider should know well enough what are the prerequisites for each exercises. In other words: what do I need to master in order to be able to execute this particular exercise?
For example, if I want to start the canter pirouette, I know that there are some pieces that I need to master independently before trying to put all the pieces of the puzzle together. I should be able to canter almost on the spot, I should master the walk pirouettes, I should have good canter half-passes. Then I can try to combine all these elements in order to try a canter pirouette.
But anyway, it is still the same idea, I keep improving all the basics until the next step comes almost naturally.
And if I try something and my horse shows me it is too difficult or he gets tense, then I know it is a bit too much for him at that moment, and I should come back to a better preparation. And when I feel he is ready, I try again and adjust, depending on his reaction.
Like everything with horses, we should always keep listening to them, and paying attention to the signals they give us...
I hope this answers your question!