I don't agree. Ethos don't grant anything, they restrict your moves.
By being neither Materialist not Spiritual, and being emperor in my domain, if I say robots are citizen then they are citizen.
Mechanically, you're wrong. Egalitarian ethics grants the ability to give the highest living conditions. Authoritarian ethics give the ability to resettle, expel, and enslave POPs. Xenophobe ethic gives the ability to enslave and slaughter aliens. Materialist ethics give the ability to give robots the same rights as meatbags. Spiritualist ethics let you unlock the ability to declare planets holy and usher in THE END.
If you grant robots citizenship, you are, at the least, denying the spiritualist side by affirming that entities whose very existence affirms that that material begets consciousness, rather than the independence of the material and spiritual or the primacy of the spiritual, as well as defying the notion that only the divine can grant consciousness or a soul. Well, that, or you're giving entities that are inherently lesser than organic life and incapable of ever becoming equal to organic life the same rights as organic life.
That said, you should definitely be able to give synthetics the exact same
range of rights as organics. For instance, authoritarians should be able to enforce the caste system on their synthetics, just like their squishier POPs