It is a bit late in coming, drothgery, mostly due to my feeling I missed the train, but here is a Republic Marine for your review and consideration as a potential PC.
Looks pretty good, but a few background nitpicks.
1 - In this alternate timeline, it's not the Imperial Remnant, it's the Empire. Which is still substatially larger than Republic.
2 - Your PC is young enough that Corellia has been independent for most of his life, and by the time he came of age, the alliance between the Galactic Republic and Corellia was pretty solid (it was somewhat shaky in the early years of the Republic). Why did he join the Republic military rather than entering Corellian service?