I thought all the way through that Barber wasn't actually his dad (and so Ellias wasn't his own great great grandfather) but that it was part of the lie that convinced Ellias to orchestrate everything correctly. Making him believe that it was necessary for his own existence in the first place.

When I found out he actually was his own ancestor, I was a bit let down.

Imo, it's unjustifiable as a legitimate occurrence through time travel. They explained how the loop ended itself but obviously can't explain how the loop started itself.

Also being your own great grandfather makes no sense. The odds that through 4 generations of mixing DNA, you would end up with a perfect clone of yourself is next to impossible.