BR-9 is highest in Sony TV line up. More expensive, even better in picture quality and sound effect.
It has more brightest among 4K range of TV which suit to user who prefer bright color than natural color.
Each has diff picture processor spec.
BR-9 is Mini QLED with highest peak luminance, slimmest panel and Tweeter sound, X-glare
BR-8 is OLED with surround sound system
BR-7 is Mini QLED with std spker.

If house living room no so big, then you may choose BR-7 enough.
Bcos seeing from eye to the TV like 2.8m from the sofa may be just fine. Too near and you will get headache when watch action movie.

It is because the horizontal level of the action scene is the one making you headache. May be ok for the young people probably their eye muscle is great condition. I don't own BR-7 but i am using older version Bravia TV in office.