Custody is the main motive and if there are other motives I doubt any of them relate to George.
At any rate I think George got involved only because he's part of the enmeshed family unit but I do think George wanted Jake to get full custody as this would be beneficial to him. Why? Because it gave his son a built in playmate and home school mate etc.. just like George had with Jake growing up. As a bonus, there might be financial gain such as through SS or an inheritance.
George also said something telling, he said Jake is always getting them into trouble over some woman and I think this implies the murders were all about Jake and Hanna and nothing about George.
I believe that before the murders George did not think he'd get caught, therefore, not be separated from his son. I think the 4 Wagners thought it would for sure become a cold case, they weren't banking on BCI and the AG's involvement or the resulting media storm including YouTube videos, Podcasts, National press, online groups bent on Justice, press conferences, etc...
I believe they were counting on the Cartel getting the blame so imagine their elation when the media picked up the story of possible Cartel involvement. Now imagine their shock and dismay when Reader said he believed the killers were locals...OOPS.........!
This is my take on it and we will get a clearer understanding of George's motive(s) during his trial and maybe during Jake's likely 2021 trial, after all, DeWine disclosed:
"The entire story as it will unfold at trial is just amazing."