Journey to Eternity and Thrashing Brontodon
Yes, the Journey to Eternity will return to the battlefield transformed and the Thrashing Brontodon will also return to the battlefield.

The exact sequence of events is as follows:


You activate Thrashing Brontodon's ability targeting Journey to Eternity. While paying the cost, you sacrifice Thrashing Brontodon, which triggers Journey to Eternity's ability.
Journey to Eternity is no longer attached to a creature, so it is put into your graveyard as a state-based action [CR 704.5m]
Journey to Eternity's triggered ability goes on the stack on top of Thrashing Brontodon's ability.
Journey to Eternity's ability resolves, returning Thrashing Brontodon to the battlefield and returning Journey to Eternity to the battlefield transformed.
Thrashing Brontodon's ability starts to resolve, but it no longer has a legal target because the Journey to Eternity left the battlefield, so it is countered by the rules [CR 608.2b]