I agree that Section 3 is murky as to how to determine who has engaged in insurrection or rebellion. At the time it was written it was pretty obvious who it was written for (anyone enlisted in the Confederate Army & Navy and anyone who had been part of the Confederate government or part of the government of any of the seceded states).
Today it defies logic and fairness that someone could be declared an insurrectionist without a trial. To let that simple unproved accusation invalidate them from Federal service or office would allow a dishonest majority to simply label all their political foes as insurrectionists thus eliminating the competition and seizing power apart from the democratic process. It certainly is not what the founders would have wanted and probably not what the authors of the original amendment intended in 1866.
Can you imagine the Framers foreseeing a case where someone would be impeached after leaving office? (Of course the purpose was to try to bar Trump from ever holding office again.)
John Kerry gave aid and comfort to enemies of the US (by going to Paris to give advice to the North Vietnamese and Viet Cong delegations) but no one used that to keep him off ballots in 2004. And he very nearly won (the Ohio electoral votes made the difference).