I've recently gotten to the Terraform Egg. I thought it would increase my Soul Egg production, but I still get most of it from Trophies and Contracts, not prestiging. I'll try getting more Epic Upgrades, but getting as high as possible then prestiging doesn't give me much anymore, at least not on a logarithmic scale (compared to what I already have). I've plotted the relative egg difficulties on a logarithmic graph, and it gets a lot harder in the late-game. Research Upgrades might help me to pull through, as well as additional, more valuable egg types.