Alright, after using two terraformers in a row with the tetractic effect enabled, I noticed that the buff in no way stacks.

My suggestion is that the time for the effect stacks, such as if you use 5 terraformers in a row, it has the effect for 10 days. Otherwise, the only good way to build up planets would be to cycle through adding terraformers to planets, making sure to add a terraformer after the effect has wore off.

Perhaps this is how you want us to build planets, and you want to keep it that way, and I guess I can see that, because I could save up 15 terraformers and 5 enumeration crystals and get a planet to 20X and have the +10% production for 30 days.

This is reasonable but do remember that 10% for 2 days at say 2x is much weaker than at 20x. That said, stacking of duration is a reasonable idea, whether full time or some reduced amount would be an issue of debate.