There is no right or wrong when choosing a data classification method. It's all about telling a story, so you just to have play with color scales and see what feels better. When you use a continuous scale and your data are not equally distributed (there are some outliers), you will get almost every country shaded with the same color except that outliers.

This is an interesting guide about data classification on maps: https://www.axismaps.com/guide/data/data-classification/

I would love to take a look at your guide when it's finished.

You can take a look at live examples here: https://datamaps.world/projects/europe-housing-price-statistics.

Besides publishing interactive maps, you can export them to high-resolution PNG or SVG.