I personally think views on a site like Jump+/MangaPlus is a good reflection of how popular a series is. I believe the view count is more reflective of the popularity than the sales.
In my opinion, the main reason Jump Plus series don't sale nearly as much as series serialized in the main magazine is because they were born as digital series.
Fans of the series likely just don't go out of their way to buy a physical version of the manga is because it's all free on the Jump+ app.
Now, granted, I know VERY LITTLE Jump+ series are popular to begin with, so the majority of them probably don't sell simply because they're not popular.
And of course every series in WSJ leak online and are available online, whether legally, or illegally. However, the millions of people who buy WSJ every week are likely people who have an appreciation of owning physical volumes. While sites like Jump+ appeal to people who are hip with reading manga digitally.
I think it's clear that Hell's Paradise, Blue Flag, and Spy X Family are definitely popular(and would probably go as far as to say that they're likely just as popular as the lower selling WSJ Main stays. But even if SXF only sells 10K, i don't think physical volume sales are that reflective of it's popularity and fan base.