I have also observed this. As soon as you engage the GG non-adjustable flat trigger or Legion flat trigger, the safety lever begins to move upwards and does so for a tiny bit before the sear is starting to move as well.
I have tried to measure the amount you need to move the striker safety before the striker can move past the block. This seemed to measure around 1.2mm~ on my gun.
I have also tried to see the contact points and what happens during pre-travel with the safety lever and striker safety. I did this assembling the gun without the barrel and guide rod so that I could see the contact points between safety lever and and the striker safety lever, and as far as I can tell with the GG trigger these two contact points nearly already seem to be touching. During pre-travel I noticed that they both started to engage nearly immediately on the very first little trigger pull.
From my observation of the trigger assembly and how it engages the striker safety I would prefer to have some pre-travel and would not try to eliminate it completely. I am unsure if on my gun having a pre-travel shorter than 1.2mm would actually disengage the striker safety.
I do not know if assembling without a barrel and recoil spring affects the distance between the safety lever and striker safety contact points. This is me purly trying to figure out how it works on my own and my findings and assumptions might be wrong.
At least for now I am trying to have around 2mm pre-travel to be on the safe side, I don't mind that anyways and actually like having a little bit of take up.