I'm trying to upgrade a project from 2017 to 2019. In 2017 a style could be set to use thumbnails however 2019 seems to do it on an image by image basis. Is there a way to set all images to Use Thumbnail in 2019?
Nope, as far as know. You can turn off and on thumbnails for the whole project in the output preset settings, but this will apply only to images that have the Use Thumbnail property set to Yes, which is specified per image. In RH2017, you still had to apply an image style other than img to images that you wanted to use a thumbnail for, which was also performed individually per image. So, it's not that different in terms of applying thumnails to images. The approach in RH2019 definitely provides more possibilities, as, for example, you can have different thumbnail size for every image, while in RH2017 it was the same for all of them.