I agree; non-spiritual psionics are far more common in sci-fi than spiritual ones. Of course Stellaris has every right to establish its own cosmology, but for a game that intentionally seems to rely on (a) referencing so many other franchises and (b) allows its players a huge degree of customization it feels like an unfortunate limitation.
I would have preferred if, say, everyone can pick the 1st Ascension Perk, but the 2nd may be limited to Spiritualists. That way it becomes more of a genetic/evolutionary thing (in that every species may get latent psychics), whereas only spiritual cultures can maintain the discipline necessary to develop these abilities further. This would not only deal with most of the customization limits, but it'd also preserve high-tier psychic gameplay as an Ethos-exclusive thing.
... I mean, I doubt robots as buildable Pops will become a Materialist-only thing with 1.5, right? Why not mirror this very sensible compromise for the Spiritualist Ethos, too?
I agree that non-spiritualists should still gain access to a (very) limited psychic tech pool. Even if it would be limited to just the psychic equivalent of the "Hyperlane Mapping" tech non-hyperlane users have that allows them to see the hyperlanes. Even materialists would see that there is something there, even if they can't really interact with it much.
I don't think non-spiritualists should gain access even to the first Species Endgame Perk. It seems like the idea behind them is that an empire will always take both, even if it's like the second and the seventh picks. Since they are mutually exclusive Perk paths, it seems like bad design to allow a player to screw their own game up by either giving up on the second Perk or forcing an ethos change.
I certainly hope AI will not be exclusive to the Synthetic Perk path, but who knows? Synthetic path does have their own exclusive thing though: converting biological pops into cyborgs and eventually synths. I imagine that is one of their unique things, rather than AIs.
I'm more worried about the Biomastery Perk path and its unique features.