Agreed with Claudia, great read and good info.
However, I'm not sold on the assertion that there is a cap on the review count vs ranking impact, though I would not be surprised at all if it gradually diminishes as you go up. I pretty regularly see businesses compete in relatively distant locations against closer businesses with lower review counts. Could user behavior and on-page seo be impacting that? Of course, but I'd love to see a detailed study on this to push me over the edge as this is a pattern I see very regularly in my day to day.
All the respect in the world for
@JoyHawkins and her team, but I have some reservations on that review keyword study as well. (this is all complete speculation, having not seen the site or it's GBP firsthand) Semantically, Google had probably already identified this business as a Christmas tree farm, and we know that keyword density is no longer a ranking factor, even for on-page SEO, so I'm not surprised that adding keywords around Christmas trees didn't work, especially if older reviews already mentioned christmas trees or if someone already said they were fresh. I'd love to see data on a business adding brand new keywords(maybe the christmas tree service starts selling christmas ornaments, or wreathes, or trees for landscaping), or featuring reviews with location based keywords (maybe reviewers mention that even though they moved to [city an hour away], they still drive all the way back to [business location city] for this business). The thought also comes to mind that maybe hidden away in Google's algorithm somewhere, there's a weighting system for seasonal businesses that impacts the ranking weight on reviews depending on whether they were left on-season or off-season. Perhaps Google is automatically suspicious of reviews left in the off-season, but not enough to hide them. Or maybe Google only re-evaluates ranking on seasonal businesses when the season opens again. Truth be told, I have zero experience on seasonal businesses so that last bit could all just be nonsense ramblings.
Anyway, please don't take any of this confrontationally! I'm just not sold, but I respect you both and I really want to be sold lol.