Ideas and suggestions.

as an alternative, you could reduce the damage of Armageddon, and when obtaining the advanced Fire skill, provide a significant bonus to the spell's damage.

By the way, a similar option was considered and may be implemented in the future – to tie the SP multiplier of Armageddon to the Fire skill upgrade.

In general, changing the cost and effect of spells for the HotA is more preferable than changing the school and level.