In fairness as my mage I too found I was using the handful of Mass and area spells repeatedly.
However, later in the game they clearly weren't as effective, especially with Black Dragons, Unicorns and other creatures that are strong and resistant to magic.
Often I would find disabling some creatures, like 500 zombies, with blind to be absolutely necessary because there was no way you could deal with them and Ancient Vampires at the same time. But with blind you have to make sure they aren't attacked unlike Sheep which costs a fortune in mana.
I think the devs have deliberately nerfed many spells. I've noticed spells like Bless become Mass only at 3rd level. But there's not enough gemstones and crystals to max out all your skills or max out your spells to their highest levels.
This goes true for monsters. You'll routinely find there's not enough castles to garrison your troops and reserves, welcome relief they are, quickly enoucnter the same problem without them. There's simply not enough space.
But this is exactly the point. This game is not Diablo - this game is all about resource management. There is simply not enough to do everything that you want. You have to pick and choose and you have to focus and and hope you've made the right decisions.
Sometimes you are going to have to drop armies completely. You can't keep holding on to them.
Some armies, like Barbarians, are tough but just plain suicidal. Your unicorns can't be resurrected and they are hard to find. Do you sacrifice a couple hundred Lake Fairies for 7 Unicorns back? Are you running out Lake Fairies you can pick up?
You can only cast so many times? A different strategy instead of Blizzarding all your enemies let them attack your Lake Fairies all day then rez 200 of them.
You are down to 1 enemy. Hypnotize them and rez your casualties. Hopefully he won't get himself killed and you'll have to find more armies, and maybe mroe cash for them.
As for economy, just you wait. If you think you have more money than you'll ever need and don't know what to buy, just wait until the resources run out. The treasure has all been found, the armies hired and all the good stuff taken - where are you going to get monsters you need and resources to increase your skills? Suddenly you find them and suddenly you are scrambling for gold again.
Its a great economy, because you you can't have everything.