I'll chime in. This is residential in Texas, 21 foot spans do not need to be sealed, but experience tells me that they do need to be engineered. The problem I see is that the full load path isn't described. I'm not exactly sure how the roof framing loads the new beam. Is there a line of support from the ridge to a wall being removed? The joists running perpendicular to the rafters means that the attic floor isn't providing restraint from spreading of the walls as the rafters try to pry the walls apart, as I would expect to see in a self-supported ridge. Also deflection will be a concern since your proposed long beam will bend between supports, but seems to be squeezed into the joist space. Since depth is essential to limit deflection, I would not try to squeeze the beam into the height of a 2x8.
I removed a wall down the center of my house, replacing it with a four foot deep truss constructed in the attic. It is holding up well (no pun intended.)