Clearly there was the classic viShNu-focal system of vedic and earlier IE provenance; then there was the more local evolution in the Indo-Iranian borderlands of the vAsudeva-sAttvata tradition. The earlier harivaMsha was definitely focused on a version of the sAttvata tradition. But there was a parallel strand of vaiShNava tradition predominant among the vaikhAnasa-s & and an early strand of pA~ncharAtra-s that was closer to vaidika focality on viShNu.
The viShNu purANa tries bring the 2 together by giving devakIputra the focus of the “earthly” expression of the sAttvata religion a central place in the narrative. rAmachandra aikShvAkava is not given much space in this text. However, it goes on to clarify that the saMkarShaNa & vAsudeva are only amshAmsha, i.e 1/p*1/q fractions of the primal viShNu. So these avatAra-s are only parts or minor parts of viShNu, namely two hairs dropping off from him. Thus, it places the sAttvata heroes in more meaningful hierarchy with respect to the god himself. But like the shuklayajurveda verse there is a certain fractality. This little part of viShNu can itself have tremendous potency as exhibited by the saMkarShaNa and the vAsudeva.
This hierarchy is the reverse of the late va~NgIya kArShNi reflex – a hypertrophied sAttvatism where viShNu has receded into the background.