I will try to recruit the initial Milira that crashlands and see what happens

I recruited her and let half a year pass but nothing interesting happened. I think probably humans cannot align themselves with Milira, but only the Skyfeather Church, so you probably have to do the Milira start to align with them.

I don't know if it helps, but I found this additional Add-On:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3477405110&searchtext=milira
It adds a few world events.
When it comes to the faction, the Church at least seems to be quite high tech level, as likely is the Milira faction (I haven't made them angry with me yet, since I only played the Milira Scenario so far)
However, due to them being quite high tech, their raids seem to only happen on high wealth levels.
But so far I haven't found any solid Milira Questline, ending or whatnot, which is a bit of a bummer, but the mod makes up for it with great content.

Thankyou, truth be told I already started playing it. Although it does seem that the game spoils you with high tech weaponry early on lol. I played as humans and yes the Milira are very dangerous. The majority of the missions are their Milian automatons which are dangerous enough as is, but I had one mission to deal with a Miliran weather generator guarded by just one Milira with a spear. I went after her with two humans armed with charged rifles, two pawns armed with charge rifles, a Knight 2 and a Paladin. She stomped us. Just zoomed in with her spear and shishkabobbed both my mechanitors, and then my milians just let her finish them off since their masters got disabled. We shot the heck out of her for a while before it, but her Valkyrie armour had crazy high armour. So yeah, they are total badasses!