With every camera I've used (Canon, Nikon, etc, etc), what it wants first is the minimum/desired ISO you set and the SS you set, while adjusting the aperture. So it is adjusting down to the desired/minimum ISO first.
But unless the light is extremely strong, the minimum ISO setting is set quite high, or the SS is extremely long, the camera generally won't be able to reach the minimum ISO... so in shutter priority the camera will seem to adjust the aperture to maximum first; especially with slower lenses. That's why I don't use the shutter priority mode (to me, the aperture setting is generally more important than ISO). And no, you cannot really change this behavior.
Instead, most cameras (like your 7Dii) are designed to work this way better when in aperture priority mode (Av). In this mode the camera also wants to use the ISO setting you selected first, but there are also more options to control the behavior/sequence. In the menu's associated with auto ISO there are minimum/maximum ISO settings and a minimum shutter speed setting (but no aperture options). And when both are set the camera will typically first use the minimum SS menu value along with your chosen aperture, and it varies the ISO (because it can't get to minimum). If the ISO does reach the minimum setting the SS then increases above the minimum value set. And if the ISO reaches the maximum setting the camera will then reduce the SS below the value set. Basically it makes the same choices I would make, in the same sequence and at the same time. And I can always override it by simply changing my chosen aperture or the desired/minimum ISO (changing the minimum SS setting requires going into the menu). Often it may be better to set the minimum ISO to a higher value such as 400 or 800; or to set your minimum SS to more of an "optimal" speed rather than the actual minimum you think you can manage/tolerate (both have about the same effect).
Another good option is manual mode with auto ISO. I'll use this when I care about a specific aperture and SS equally; like when forcing a long SS for panning.