Shinryu, my guess is that Ūryu initially joined the Vandenreich to kill Yhwach and avenge the death of his mother. However, because Yhwach is an extremely savvy fucker (even by
Bleach standards), he decided to limit Ūryu's actions by making him Sternritter A (giving Ūryu his [Yhwach's] own letter in the process) and his successor to the throne. This decision upset the Sternritter, which caused them to take Ūryu's every move into extreme scrutiny, which combined w/ his extreme proximity to Yhwach essentially leaves him powerless. There's also the fact that Yhwach might be curious to figure out why Ūryu, a Gemischt Quincy, survived his Auswählen.
As for Ryūken's disappearance, I think that's because Kubo has not found an ideal opportunity to bring him back to relevance yet. Yea, his son is in the Vandenreich and has even been selected to be their leader's successor, but Kubo has yet to explain why Ryūken (and Sōken, by extension) decided to keep the existence of the Vandenreich a secret from Ūryu for as long as he did. I feel that Ryūken will be relevant once Kubo explains that.