First, all this can be done in hand, or at liberty with mimicry as you've already used.
Oh my goodness he has a lovely piaffe already. It is NOT marching. It is buoyant and pretty. The piaffe is a lifetime achievement, meaning it is worked on and improved over the lifetime of the horse. It always has a small beginning...some good, some not so good, but it improves as strength improves. Your horse is already bending the haunches and has found a balance that works for now.
For you, I would definitely say that your passage could come from your piaffe...simply take the piaffe forward and a reward a few more steps forward. If your piaffe doesn't end in a halt always, you can develop the passage from a more forward piaffe. No problems.
Do not worry about "fuzzy aids". The horse will figure out in time that (for instance) the same set of aids can request either school walk or spanish walk. It's all in your energy and intent, which horses can read clearly. To an onlooker, it might look like you are not asking any differently, but your horse will know. Expect a little confusion at the outset, but as long as you are calm and reassuring to the horse (ask little, reward lots), your horse will remain calm (or quickly return to calm) and will trust the process with you. Piaffe and Passage require "positive tension" in the horse. That tension is GOOD tension and the horse can remain mentally calm.
So....I think all of this is to say that you already have the basics of your passage at your fingertips. Over time you will work on the balance and cadence and the rhythm...trust that it will grow into everything you want it to be. It is never pretty at the outset. It never springs to life from nothing to perfection. It grows gradually to perfection through practice and love
In spanish walk, if you get just the right amount of forward, your horse may not stomp so much. In the beginning that forward is sometimes at the expense of amplitude, but the amplitude will come back. If you are using a target for the spanish walk, you can present the target and softly move it away as the horse reaches up and out and in time (it always takes time) the horse will reach out more with each step and will get quieter about it.
Again, it is always a process of a beginning (sometimes not so pretty), a middle, and an end. Sometimes the middle is very long. All of your work combined will affect everything, whether you work on it or not. If you didn't do a piaffe for a year, but continued to work on other gymnastic exercises, you would find the piaffe improved (if a little rusty).
So I would say for you, not to over-think things to much
You have a very nice start on both piaffe, and the gateway to passage.