I also want to add that while i'm happy to see us rebuild our depth in the QB room (and I think it is in a better spot than it was last year), I still think Sims is likely the clear starter.
I don't see Phommachanh as really overtaking Sims, at least not early in the season.
He is coming off an ACL injury from Clemson's 2021 Spring practice (though he still played in 5 games in 2021 for Clemson that fall). But his stats against ACC teams leave alot to be desired.
In 8 games against ACC opponents over 3 years (GT(2), Syracuse, FSU, L'ville, Miami, Wake and Pitt) he is 8-23 passing for 45 yds with 0 TD's and 1 INT and has 22 rushes for 99 yds. Now he is clearly playing in mop up time so the offense is going to be set up for him to run more, but those passing numbers (which was considered his weakness out of HS) are not encouraging.
He looks more proficient against lower level teams. In 4 games against SC ST, UConn, Citadel and Charlotte he is 14-25 for 159 yds passing with 1 TD and 2 INT and 8 rushes for 51 yds. The majority of those numbers come against SC St.
If you believe Sims is the clear starter right now, and I do given we have seen nothing indicating otherwise, then I would say Gibson appears to be the better backup in terms of passing and Phommachanh appears to be the better backup in terms of running.