Google has always downplayed the importance of priority and chance frequency in the XML sitemap files. They have told webmasters to not worry about it and even said they ignore other fields like the lastmod date.
That being said, Google's Gary Illyes put it to webmasters another way. He said that the priority field in sitemap fields are "essentially a bag of noise."
So you really do not need to waste your time filling it out. You can just move on to more important things, such as bulk quality stuff.
Here is the tweet:
@denstopa we ignore those. It's essentially a bag of noise— Gary Illyes ᕕ( ᐛ )ᕗ (@methode) March 28, 2017
Forum discussion at Twitter.