This comment keeps coming back into my head, and now I'm worried about doing anything with Stephanie at all. Can you be more specific about what you can or can't do with Stephanie before Ellie shows up (if you don't want to lock out Ellie's events)?
I don't believe Stephanie is tied much to Ellies events. I'm using the beta version, but I just went through both Steph and Ellie files to find any relationship between the two. Ellie is mentioned only once in Steph files about the teamup scene, and steph is mentioned a few times but nothing should impact Ellies events.
To unlock Ellie, you have to first unlock and upgrade the Nanobots which requires Tier 1 trait unlocks (upgrading research once with Steph) and you may need to corrupt Steph a bit in the first place. After that, the events surrounding Ellie will fire, you can't miss them nor can Steph block it. The only way to miss her is by not having an open employee slot, which the game very loudly warns you about ahead of time.
There are only two other times Steph appear with Ellie, once for the teamup (threesome scene) and once for Ellies obedience path. However, in the obed path it's Ellies stats that matter, not Stephanies.
My guess is that the original commenter doesn't realize how to get Ellies initial events to fire. I'm like 80% it's related to her lust score, so it can seem a bit random at times. She also may need to be upgrading the nanobots specifically, not the office upgrades.