I am finishing Floating rocks level right now. I found all camps, caches etc on map and all what is left is are territories without special items and waiting for camps to deliver armies. There is no point in doing any economy, as army costs rose so high that it is pointless to sell anything and gain more money as I have no effective use for them.
All can I do now is check every two hours, conquer one of two territories with and wait two hours again. This is really not enjoyable in the slightest.
A common mistake players make is buying mercenaries too early.
Think of it this way: You're spending some money on mercenaries and some money on army camp upgrades. Buying mercenaries gives you armies immediately, but buying army camp upgrades gives you armies over time. Therefore, it's far better to do those army camp upgrades first and buy the mercenaries later, so your camp upgrades have more time to produce armies. Upgrading army camps when a level is almost complete is not an efficient plan.
When to buy mercenaries is a balancing act and there's no sure-fire rules on when to do it. Ideally, you want to be purchasing your last mercenaries to conquer the last territory on the map.
The speed at which you progress through Warzone Idle is based on your strategy, so if things are going too slow you should try revisiting your play style.