arches doesn't take place after any of the echo routes
Also TJ and Flynn have one ending
not really, people just over complicate the timeline shit and that isn't entirely the fanbase's fault since this game encourages using context clues and deep analysis to understand certain aspects better (especially the characters).
The way timelines work in this game is the same as any other vn with multiple routes but no cannon one. There's no cannon route in echo, they work together to tell a single story and arches decided to tell a story that takes place in neither of them. The devs confirmed that the events of echo did happen before arches with the crew and all, but not in the way it did in any of the routes.
wait a minute, haven't creators themself said that Echo and TSR are multiverse and every route indeed happened?
That is why there is no "true ending", because every single one and none at the same time are those true endings?
Edit: so, I looked a little in discord q & a and actually I had asked this very question about Echo's multiverse. George said that every ending is canon and happens ;)