I think Basic training is a limiting factor, but I agree that there are other (maybe more important) limiting factors in getting his army in shape.
In my opinion it is still possible to improve other aspects of training via the spiritual realm boot camp. Remember, Taravangian did manage to send the entire Karbranth to the spirit realm.
All Tarabution needs to do is get some war leaders that are loyal to him (like the Blackthorn) and work with them to set up some sort of spiritual realm boot camp, he just needs to fake another tidal wave or something to cover up sending a military school and barracks to the spiritual realm. This should only take a day or so. Then Tarabution can send a whole bunch of people there covertly, maybe send lots of smaller groups to begin with, Then send the whole lot of them to get them working as an army.
Considering Big-T aged Gav to adulthood in a few hours this whole process shouldn’t take that long and he probably doesn’t need to give too much attention (the Blackthorn will get that army in tip top shape). Even if the time dilation makes it take longer 10 years should be plenty of time.
If I were in Big-T’s highly invested shoes this would be my plan…
There is a larger issue here in my opinion… I really enjoyed Wind and Truth, however, the magic system is becoming more and more “soft” (the shards powers are unclear, confusing and can easily create inconsistencies). Originally I was attracted to Brandon Sanderson’s books because of the Hard magic systems, his magic rules were simple, and this was good because it made sense, the way the magic users interacted with the world made sense.
There’s nothing worse than having a story where all of a sudden the wizard that was travelling with the party does a super powerful fire magic blast out of the blue and destroys a whole army of baddies. And I think “where did that come from? Why doesn’t he just do that all the time? Why do the good guys even make armies? Why not just have that wizard guy win everything??”