If Fani (or her replacement) gets a conviction before 1/20/2025, it would be up to the current governor. Since he seems to have set the stage for Fani by interpreting an innocent remark by Trump as an encouragement to steal Georgia’s electoral votes, I wouldn’t count on him to give Trump a pardon, and he doesn’t have an election to worry about.
Even if convicted before assuming office, I don’t think a state could legally hold the US president in prison during his term. Another poster went so far as to call that an insurrection. But I doubt they would imprison Trump if elected for a few days/weeks with the appeals process still working it’s way through the courts. So I think it would be the next GA governor that would ultimately decide Trump’s fate should he be convicted, not pardoned by the current governor, and lose the appeals. (But I’m no lawyer.)