Research has shown that the ideal line length is about 65 characters. Anywhere between 45 and 85 is generally seen as acceptable, in the context of a roman alphabet.

The solution:

.wrapper {
  display: grid;
  grid-template-columns:
    1fr
    min(65ch, 100%)
    1fr;
}
.wrapper > * {
  grid-column: 2;
}
.full-bleed {
  width: 100%;
  grid-column: 1 / 4;
}

ch is a unit, like px or rem. It corresponds to the width of the 0 character in the current font, for the specified font size.