The default reaction to our ancient story is "You're a fucking liar and a heretic." We are not in a position to prove our benevolence despite not having been born to the imperium, and while the planet will still be there next turn, so will their memory of us snubbing them
now.
Don't confuse "worth diplomacy" for "worth being friends". Their administratum has records of the local system and traffic through Ascalon that we want, the ecclesiarchy and sororitas has information about the imperial faith we want with Neablis saying it could be relevant to our faith studies research outright:
Holy artifact samples to study. Faith superpower users to observe. New imperial tech (power armor especially!) to reverse engineer (and they'll effectively be paying us to do it!). Administratum records to give us a clearer picture of the neighborhood, and getting them used to and a positive impression of the tech base of their neighbors (Denva).
That's plenty of reason to want diplomacy. Diplomacy does not mean friendship - sometimes it's just transactional, and I think Alectai/Meian's explorator sponsor story works best. From our position of power all we have to worry about as far as harm they can do to us goes is our reputation, and I think said write-in works far better than the Ancient story as well.
Anyways,
@Neablis hasn't said what we'd call ourselves if we don't write anything in, I think.
Lastly -
Plan: Blessed Shields and Holy Relics (Are For Trading) takes advanced implants with Anexa, setting us up to get the HSI tech next turn easy squeezy. We're fine on that end.