Your using a $3k+ camera but a kit that costs around 10% of what you paid for just the body of the camera alone is too expensive? Poor sound quality makes even 8k (it exists) pictures look amateur
Check out the ZOOM HN4 and ZOOM HN2 both can do what that kit can do at 1/3 the price.
You can us the aux in on the hn2 and record directly to the card, mix it later in your editor. Im sure the sound cards of those mixers are better than the a7rii
OTHER THOUGHTS:
Long cheap aux cables make all kinds of noise and pull in static/radio interference im not sure how far away you are from the preout but you could try upping the quality of the cable your using and see how that works.