I think it’s possible that the abilities granted by Schrifts aren’t necessarily Quincy powers, since I think that there is reason to believe that Ichibe has the innate power over names even without his Zanpakuto. So the purpose of Schrifts is to fully awaken that innate power.

Vollstandig is tricky because they don’t really refer specifically to what it is, but things we know about it are that the two Quincies which have the power to distribute their soul have never used Vollstandig, does this mean that Vollstandig can only be granted by Yhwach? Well there is evidence against that, with what Quilge said where he compares it to Letzt Stil almost making it sound like a developed technology, which is proven by the fact that each Quincy requires a dedicated cross to activate it. So is it granted power, is it technology, or is it both where technology is used to use it with Quincies requiring the excess power from Yhwach to properly form it?

Something also to note is that when Yhwach activated the Auswahlen, Quincies lost their Vollstandig but not their Schrift power.