Science/philosophy/religion go through an Enlightenment, the non-noble (and not-powerful minor kizier lines) push for more equitable government, several kingdoms collapse as kiziers lose their shit, solar-magic and then solar-electric power is developed so people rely less on kiziers and their power becomes less impressive. Democracy develops, kiziers are regarded as dangerous and unstable, their power slowly fades as the aristocracy in Europe has, they're largely still wealthy and can do a lot of fine magical control jobs that are hard or expensiv
e to replicate with science but no longer in control.
Once a kizier is matched to a tharii they're basically a ward of the tharii and considered a single unit for legal purposes; if they're not paired, kiziers are either the responsibility of their family or placed in convent-schools. The occasional group of unPaired tharii from the dark ages who hunted out of control kizier have become formal orders, often with vows and spells to keep them from Pairing. They learn to use their tharii status to control unPaired kizier.
The French Revolution sucked for Kiziers.
During this period there's still a strong strain of the kizier families "watching their own" and doing their own matching/coming out parties but as the Enlightenment goes on, events like the French Revolution and the decline in power start shrinking kizier familes and thinning the ranks as it were, but reincarnation being what it is, those souls have to go somewhere...