Many thanks for playing and posting your insights.
The progression is largely linear with the exception of the enemy gen rate which is exponential, partly to ensure finite playing time and partly to allow new players to find their way. I sacrificed difficulty options to meet the jam deadline :(
There is however some randomness in the allocation of each upgrade type which is intended to offer a little replay variety since distribution affects strategy somewhat. The upgrade gen rate is constant but in the late game you have to juggle harder to continue gathering new upgrades whilst doing battle :) My guess is the spikes/lulls you see come from certain upgrade type biases affecting the balance.
It's true that if you last long enough it's complete mayhem ;) Thanks again for taking the time to give this feedback.