Is the game really designed to be played idle?
The game description says yes, but the game mechanisms say no.
I'm at level 132, and from what I see, this game strongly penalizes long idle periods by having storage limits. And even worse, in the horde, I have to constantly buy upgrade to be able to make any progress. The farm also requires very frequent planting.
This problem is especially significant in the horde. After ~Z36, I basically run out of bone upgrades to buy. The infinite upgrades alone are not enough to keep me going. So I can't get any further, and the only option is to prestige. This means a long run isn't much better than a short run.
In a short run, I can reach Z38 in 40 minutes, earning ~120 soul. If I keep pushing, I can get to Z41 in half a day, but the reward is only ~180 soul. This is nowhere near enough to make long runs viable, and the new unlocked upgrade is also unaffordable.
From what I see, the only way out of this game design disaster is automation. However the only automation are mining autoprogress and auto horde boss. Not even autobuy upgrades. And many mechanisms are designed in a way that is very difficult to implement automation.