1. It's kind of rough that the monster level increases on its own. If the player could control this (choose when to increase monster level in order to increase rate of essence) that would feel better.
2. It's not great that Arcane/fire are supposed to become obsolete over time since their upgrades are capped, especially since you can spend skill points to upgrade them. It would feel better for them to remain relevant, not just for skill points but also to feel more fun.
I think both of these suggestions would also help with other complaints around pacing / progression slowdown.
Other than that, I had a good time and the game was fun, so I am hoping you continue working on it and improving it.