If they do manage a conviction, and Trump is like "nah fam I totally pardoned myself back when I was president!" and the supreme court upholds Trump being able to pardon himself (I don't think they would; the GOP has been looking for a way to cut themselves free of Trump and this would be a very clean one) I imagine that the conviction would be a pretty damning first swing at convicting him of criminal election tampering in various states, which... yeah, he can't pardon.