It’s intentional why the primal mounts are not available for purchase immediately when a primal is released (even for a higher cost), because it keeps players like yourself and your girlfriend going into the content. That, in turn, allows the Party Finder to remain active, and players that didn’t farm it 99 times on release can more easily get into groups (because people are actively farming the primal, even if they have already surpassed 99 totems).

Even with you and your girlfriend having 200+ totems, you are still participating in the content. That’s what the developers want. It’s artificial longevity, sure, but you’re keeping the content alive longer.

As for when Seiryu’s kamuy will be purchaseable, Zurvan’s was made purchaseable in Patch 3.56, which released on March 28, 2017. So, if they follow the pattern from Heavensward, Seiryu’s will be buyable whenever Patch 4.56 drops—which will probably be around late March/early April 2019.