I have a hell of a hard time answering this because of The Massive Cliffhanger that is the end of Gallifrey. And it doesn't help that the Benny audios seem to be setting Brax up to lose the Collection completely. AND. AND. AND. In the Benny books, there are bits where it's noted that all records of Brax, Benny, Jason, Bev, Adrian and the rest have been completely wiped by about forty years from now, and that apparently the Collection no longer exists. Which is just a major "Wait how is that even possible?" Indeed, the end of one of them says that after The Wake, Braxiatel removed all records of Benny ever having worked for the Collection, which is just completely improbable and impossible.

Spoiler for S5 of Doctor Who: Brax is controlling the crack and using it to sweep up continuity errors. And also to give me a headache!

Okay, so my "OH MY GOD. I DON'T KNOW. WHAT. THIS IS RIDICULOUS. AUGH," aside, Brax would have been with Romana for the Time War, because you may or may not have noticed but he's a little attached to her. Soooo, what do you think happened to Romana during the Time War?