It would be good to replace the rewards for completing quests with a unit of the faction's race, rather than elementals. And phoenixes as a reward for Ash are not very fitting.

I had an idea about using units of the faction's race, but it's impossible to implement in the map editor without using scripts. Elementals don't have a morale penalty with either good or evil races, which makes them ideal as a reward. Besides, if instead of elementals, there was a level 4 faction unit stack, some races or heroes would gain an advantage, while in the current version, this stack is just a disposable unit for the early game.
Regarding the "lack of seriousness" of phoenixes: I'm even a bit confused, as I initially thought that this alternative was even too advantageous compared to Ash. What do you suggest replacing the phoenixes with?