OK OK OK. SO. I HAVE AN IDEA. So we want Ryland and Colt to be brothers around the same age yeah? AND for Ryland to go to space and get back before the events of The Fall Guy. Also, you said scientifically possible right? So... what if they were only twins by a technicality?
Ryland Grace is 17 years old when his baby brother is born. He instantly becomes a great big brother, playing with and taking care of him in his spare time, despite soon after becoming busy with college. When Colt is 8, their parents have an accident and end up dying. And so, Ryland takes his little brother in. He promises Colt he'll never leave him. Unfortunately, he isn't allowed to keep that promise. Colt is swept up in the foster care system, where he fairly quickly gets adopted by the Seavers (distant family of Eva Stratt, she pulled strings to ensure his safety as soon as she had a spare moment)
Ages when Ryland is sent to space:
Years between on earth: 25 ½
Years between in space: 8 ½ (4 traveling to Tau Ceti, six months solving the astrophage problem, 4 traveling back)(due to time dilation the travel time is condensed basically, it's complicated and I don't fully understand how and why it works. Like I kinda do, but also not)
Very important note: there was NO Taumoeba leakage. Rocky is fine (just missing his bestie) and so is Erid
(Ryland sends the beetles ahead of the Hail Mary to ensure earth is saved as soon as possible. The beetles are able to travel at twice the speed as the Hail Mary due to their size and lack of passengers. Thus, Colt is 31 when the beetles arrive, the sun begins to heal, and Ryland Grace is hailed as the Sol Savior of the world. The beetles arriving and the sun starting to restore also means the film industry picks up a bit more at this point, though it was still going during the astrophage crisis due to the human need for escapism and entertainment.)
Ages when Ryland comes back:
Ryland: 37 (due to the time dilation)
It would be so interesting to see Ryland and Colt navigating their brotherly relationship together. Ryland never got to see Colt grow up, in fact the last time they saw each other Ryland was Colt's guardian, and now they're equals. So that's gonna cause friction
I have more ideas for this AU concept (including elements of Starstruck), but it's past 5am and I should probably sleep... (BUT I DON'T WANT TO.... But I must.... How about I try to sleep and if I can't I'll come back? Yeah? Yeah.)