The rules around elections are very specific. Running for president provides Trump no protection from prosecution at all. He’s only protected for the 60 days in the run-up to his own election.
However, Trump is an “ordinary” citizen in name only. Let’s be honest: In terms of proceeding with a prosecution, he’s not. Garland has more responsibilities in processing this one.
I don’t think Garland will wait long past the Georgia race. I expect he’ll indict sometime in December. Obviously I have no inside track, but I can understand why he’s waiting for now.