Just started Shadowbringer but I have a small question (spoiler)

[Enkidoh]

I'm pretty sure that I remember the exarch stating pretty plainly that the reason we came here with our mortal bodies, all of our equipment, and can travel back and forth at will using aether trails from aetherites is because we were the specific and intended target of his teleportation spell, thus the spell was able to work correctly and in its entirety.

The reason the scions only got their souls teleported, and cannot travel back freely, is because they were not the target of the summon spell, they were accidental targets of a spell that was intended for you, and as such they have these rather unfortunate side-effects.

It doesn't have anything to do with him "practicing" or "mastering" the spell.. He just finally got ahold of his intended target, and so the spell was able to work fully as intended.

Although we were indeed the intended sole target of the Call spell, and that the Archons's souls being called instead were just unlucky accidents, the Crystal Exarch actually states that the spell was actually "difficult to master" and that Archons were drawn there accidentally by his "fumbling hand", which all sounds to me like it took him ages to master the 'art' of calling the player:

Crystal Exarch: "My intention had been to summon only you, but the art of reaching across worlds has proven exceedingly difficult to master. Thus it was that my fumbling hand closed upon those to whom your fate is most closely bound as well."
Crystal Exarch: "As they were not the object of my summons... their transferance was... incomplete. Though they may appear to possess corporeal bodies, they are, in truth, merely spirits one can see and touch."
Crystal Exarch: "Consequently, while you yourself will be able to pass between worlds with relative freedom, they will not. Much as it grieves me... they are stranded here, unable to return.

It's why the Call spell finally worked for us, through trial and error he finally worked out how to do it properly, and bring us physically to the First.... albiet at the cost of the Archons now being stuck in the First as corporeal spirits....

Either way, the Call did not seize the souls of the Archons one by one as a means of getting to us as well, they were accidental caused by the Exarch still learning how to do it (he was basically winging the whole thing, experimenting and finding his way with things), and it paid off eventually by succeeding in drawing us to him, and ultimately, in body as well as soul, to the First proper.