Ideas and suggestions.

My suggestion: make troop transfer a 4th-level spell. In my opinion, it's largely comparable to a city portal; it's harder to compare it to teleportation. Here, it's a city portal without the need to actually go to the city. Essentially, the army is taken from some central location (city or main hero) and sent off. Only the mobility of an individual hero doesn't increase, but the overall mobility does. Forbid transferring troops "from oneself," only allow transferring them to oneself at all levels. In total: a weak hero must learn advanced fire magic to be able to receive troops from the main hero, and the main hero must possess it as well to receive them back. This still introduces some limitations and the need for minimal development of secondary heroes.
+ possibly, it could consume part of the turn (or was that implied?)
The spell that grants + movement points near water, which was also suggested, requires a limit on the number of uses per turn. Otherwise, one could think about a deadly combination of such a spell with teleportation from the air...
P.S. I'm not a particularly skilled or consistent player in the third game, so I may be wrong about many things, but the ideas being discussed are very interesting, so I wanted to chime in.

Thus, weak heroes will teleport troops to themselves, and before the end of the turn, the weak hero near the main hero will teleport the troops to himself and transfer them to the main hero, who may not need to learn this control spell.

Heh, I didn't think of that.