Body Purism doesn't care about replacement limbs for FS advancement. Implants, piercings, ears, tails, horns, facelifts, height or hip width surgeries, virginity, tattoos, and makeup. It's a really easy FS to play...just keep all your slaves unmodified baseline humans. In my experience it's FAR easier than Transformation Fetishism, which requires spending tons of money on turning your slaves into unrecognizable abominations.They don't like cybernetic limbs, either, be they the highest or the weakest, any replacement parts cause negative reception.
Pregnancy biometrics collars mark a slave as "visibly pregnant" if she's pregnant at all (even on week 1 of the pregnancy). That's what they're for. What kind of Repopulationist keeps half his fertile slaves not pregnant? That's your problem, right there...(edit: for numbers, 16 extremely pregnant girls out of 32, everybody wearing biometrics, gets ONE point from the collars. The broodmothers get +15, and everyone not visibly pregnant racks up -32. Everyone not pregnant counts at least twice as hard as a broodmother (30+ babies).)
ALL of the FSes are easily sustainable unless you have neighbors actively fighting your choices with influence. Revivalisms are fairly hard if you have neighbors with opposing Revivalism that is more advanced, but all the other ones can be sustained even against social influence just by having enough compliant slaves.
They're almost all also "so restrictive that any small deviations will cause implosion" as you put it (except for Maturity/Youth Pref, Pastoralism, and Antebellum Revivalism, which actually ARE super easy, since they have no per-slave penalties, only bonuses). Even the ones you like are that restrictive...they just happen to align with YOUR play preferences better. When you pick an FS, or ideally even before you pick it, you HAVE to "commit to the bit."
By the way, this is exactly why before you adopt an FS the game tells you how hard it thinks it will be for YOUR arcology to sustain. If it says "Resistant", maybe don't pick that one. Go for the ones that are "Primed" instead, because they're the ones that your playstyle so far has leaned toward.