You could try allowing new vortexes to be aimed on their first turn, giving some reason for conjuring them in the first place. This unfortunately sums it up, lone magicals left to their own devices will fade away back to their own continua. Combat is by differential, using effectively a different table for each pair of colour relation ships. The form of the combat results table makes it very difficult to successfully attack units on hexes of their own colour, and if you can work up some sort of parity of forces, losses for both sides will be about even, and usually only about one loss point each. Since depletion can be 'repaired' by sorcerers in their next movement phase, by the time the next combat phase comes around the defending units are back to full strength and can repulse the attackers.
Fortresses especially are virtually immune to attack.
Some units, however, are relatively easy to destroy, to wit, lone magical units. In fact, the whole system seems engineered to prevent magical units from exercising any independent action. They usually are not worth the trouble. Cloning allows multicolour sorcerers to split down into two or three one colour sorcerers. It is hard to think of situations where this is worthwhile, even though it does enable you to convert more white hexes to colour.