great post!
one major issue i see in reviews is that a lot of people have no incentive to post a honest review at all. either they post a 1star rant review or they post a 5star brain-afk review for getting a coupon.
in our company we force our technical procurments team to write one good and one bad thing about every supplier used in a project. and also give an overall performance mark (1best to 5worst). it doesn't matter if we had 1 or 100 orders for this project at that specific supplier. we need this for our iso certification.
nevertheless the comments seem to have no real value at all, as they either write "-" or a wall of text no one reads. the grading depends actually on the people. there is one group who almost everytime gave a non meaningful grade (3,3,3,3,3) and the other group gave extreme grades (1,5,5,1).
however we change the process so that the team has to grade during the process (after the order, after the incoming goods inspection, after the component is installed) simply by giving a 1 to 5. i know it doesnt solve the problem of subjectivity but it gets us closer.
tldr.: think about: incentives reviews, directly integrate them into the process, normalize reviews