I've used Averia (Serif Libre, specifically) for at least a decade as my primary font for email, web pages in 'reader' mode, writing long-form text, etc. I find it extremely legible, and even calming.
Ironically, I've been a typographer for decades, both for print and online. Averia might seem an odd choice for someone intimately familiar with typographic theory/history and the vast catalog of possible fonts. But there's a certain pleasure and comfort in a font that is not trying to stand out or do anything particularly special.
The trick is that there are two "averages" in play. Let's call them the "attractiveness average" and the "physical average". Your intuition concerns the attractiveness average: you know that there are beautiful people and ugly people, and "average people" must be somewhere in the middle, yes?
But when scientists average faces to create a perceived attractive face, they're averaging together the physical characteristics of each face: distance between the eyes, position of nose and ears on the head, size of mouth, symmetry, etc. The claim is that we have an intuitive, perhaps instinctual, notion of what humans "should" look like and our perception of attractiveness is roughly a measure of conformation to that standard. So an intuitively "average-looking" person is more correctly stated as having a medium amount of deviation from the human mean.
The main reason it has this "calming" feature is because it's imperfect. By averaging different, sometimes incompatible font faces the result looks like a letter pressed on a soft paper, with all it's natural imperfections. It looks real.
Somehow I was not aware of Averia and used Old Timey for exact same reasons in the past.
On the other hand, someone here mentioned "Lato", which to me looks exactly how two robots would write holiday postcards to each other.
Ironically, I've been a typographer for decades, both for print and online. Averia might seem an odd choice for someone intimately familiar with typographic theory/history and the vast catalog of possible fonts. But there's a certain pleasure and comfort in a font that is not trying to stand out or do anything particularly special.