Hanna, it makes perfect sense because the two halves...the front and the back can be disjointed or disunited. In fact there is also that third important part...the mind. This I think, you have covered very well and already know how to tell Amiro that you admire his pride and his strength.
So how to get the back end (the collection engine) to assist or drive the front end (the ramener or relaxation engine)?
Goat on a mountain, and the lateral exercise. The lateral work - the shoulder in and leg yields while in ramener, help not only to strengthen the driving engine, but also help strengthen the stretching and relaxation capability of Amiro as well.
But you should keep doing it all...a little here, a little there and keep it all mixed up, not concentrating on any one exercise too much in one session. Have you taught Amiro the Goat on a Mountain? I can't remember.
But you can ask him at a stand still to first lower his head to the ground (or nearly so), then to step under from behind just a little, then ask for ramener. Then walk out of it.
This is a wonderful stretch and will give Amiro a feel for the rounding of his body. Over time, you ask him to step under further (literally, until his back feet can touch his front feet). Still with his head very low while he sets it up. This is really quite easy for Arabians. They are very flexible and the shorter back makes this easier as well. This of course relies on him not having any injury that would prevent him from doing it.
Also over time, once he has learned to step under completely, you can then begin to ask him to raise his head more when you ask for ramener. At first he will need to keep his head very low, but over time he will be able to raise the base of his neck more and more. So in the beginning, his head and neck are curling under, well behind the vertical. Keep an eye on his balance so he doesn't curl so far that he falls over! And only ask him to hold it a few seconds and then walk him forward out of it.
As Amiro develops his Goat, this will mean that you are developing a cue to cause it to happen. So then you can ask for it, ask for ramener, walk forward for several steps and then ask for the Goat again. So he stretches forward into the walk, down into the goat, then forward into the walk, etc.
In time, you can then walk along beside him, ask for ramener and ask him to try to do a Goat while moving. He won't be able to, exactly (not at first), but you watch for any attempt by him to simply step more under himself as he walks (because you have asked him to), and reward for it. This takes careful observation, but dont' be afraid to reward even if you're not sure he's actually tried to step under more. It won't hurt anything!
I said long ago that I train both ends and work toward the middle. I still think this is exactly what I do. At some point the back half connects to the front and you get a Goat with Ramener and eventually into movment, it becomes an attempt to piaffe or passage.
If you watch my little Goat on a Mountain video
You will see that Tam drops his head in order to step his hind feet under. Then when I reward him, he raises his head to get the treat? This works really well!
And since you have all the patience in the world, I have no doubt that you will get it in no time.
It's all about taking all these little single behaviors and playing with ways to put them together.