My last post referred to the Youtube app on Roku TV. Unfortunately, setting the quality on one video on their site doesn't change the default quality on all the other videos. However, I found a chrome extension, Auto Quality for YouTube, that allows the viewer to set the default video quality. This extension works great.
I think that the video quality can be changed on Tubi as well, definitely on their site. I'm not sure about their Roku TV channel, however.
After making some of these data saving techniques, I am now averaging 5 gigs of data per day, well below the cap.