Third GA attempt ruined by purifiers. How do you all do this?

Well, my third attempt at a Grand Admiral (no scaling, difficulty adjusted AI modifiers on) has met another end at the hands of fanatic purifiers. Playing as a Fanatic Egalitarian/Materialist void dweller empire with meritocracy/parliamentary system.

What am I doing wrong? Even if I neglected building other habitats, trying to ramp up alloy production high enough to counter the AI’s production feels impossible. Leaning heavily into unyielding and choke points have helped, but I still get ground down.

I’ve even rerolled several games to avoid purifiers as neighbors, but even then they usually eat their way to me 60 years into the game.

What am I doing wrong? (Besides playing void dwellers).