Can't figure out what I'm doing wrong

[Taqwus]

Early on: yes on mercenaries, not temples.

Being able to recruit a priest with fire/holy leading 100 pikemen for in the whereabouts of 600 gold (AIs are not generally aggressive bidders) in one turn even in the lowest-resource province can jump-start your expansion. Sure, you're renting them. Renting troops to take the initial charge of independent knights, however, might end up being cheaper than sacrificing national troops.

It should also be noted that some Late Era nations will have large armies full of low-quality troops -- don't bother trying to match the Ashen Empire or the Dreamlands (Late Ermor and R'lyeh, respectively) in quantity. Crush them with quality. The AI in general will also pay much less attention to research and mage support... or supply... then you might, too.