After more testing, I've determined that housing quality does not have any bearing after all. It's instead health coverage that is the missing element.
In the Massilia I tested (the one that I thought had too many casas), walker opinion can be quickly improved by adding a single clinic to the block of large casas north of a long bridge. Of course, doing so will raise the city's health to a higher level than its creator(s) intended and possibly ruin the demographics, but the point is that this turned out to be a consistent and reproducible way to improve walkers' opinions, provided all other conditions are met.
Brugle, in your Massilia, walkers will stop giving their highest praise if you so much as delete one clinic (although this won't happen immediately because you need to wait a while for the clinic coverage to gradually decline). If the clinic is instead substituted with a hospital, this doesn't happen. Furthermore, if a building receives both hospital and clinic coverage, it is counted twice, so a house served by both can compensate for a house that's served by neither.
As for the calculations, I wouldn't be surprised if the game simply divides the number of houses receiving some form of health coverage by the number of houses in total. If the final value is not fairly close to 1 (or even >
=1, but I'm not sure because I was lazy and didn't feel like counting every single house with health coverage and adding up the numbers), then the city will not have met one of the conditions for receiving the highest walker praise. While a value of 1 may seem overly stringent, keep in mind that houses can be counted twice, so there is certainly some leeway.[This message has been edited by TheNoseKnows (edited 01-27-2019 @ 04:31 PM).]