Ideas and suggestions.

My suggestion: make troop transfer a 4th-level spell. In my opinion, it is largely comparable to a town portal; it's harder to compare it to teleportation. Here, it's a town portal without the need to actually go to the town. Essentially, the army is taken from some center (town or hero) and sent off.

Only the mobility of a single hero does not increase, but the overall mobility does. Forbid transferring troops "from oneself," only to oneself, at all levels. In total: the 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. Still, there are some limitations and a need for minimal development of secondary heroes.

+ perhaps it could consume part of the turn (or was that implied?)

The spell that gives + movement points to water creatures, 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...