Great post, and despite the fact I voted Truss I totally agree with you. I shall justify my vote for Truss by saying she did the most damage if you pro-rata it on a daily basis.If we're looking at damage done per day in office, then there's only one winner.
However, Truss was such a flash in the pan that history will view her as an irrelevance, or at best as an interesting statistical quirk given how pathetically short her premiership was. A lot of people are (understandably) voting for her due to the direct financial impact she had (and is still having) on them through higher mortgage rates. The truth is that at least some of that increase would have come anyway, yet that awful mini-budget gets the blame for the whole hike. The problems she caused will be mostly ironed out in the medium term, so her long-term negative effect on the country will be relatively small IMO. That said, she was entirely unsuitable for the job and I can't think of a good thing to say about her time in office (and even less since).
For me, it's Johnson.
The damage he has done to the whole institution of government will be long-lasting, both from the perspective of those working within it and the public's perception of it. All done with a smirk at the behest of his financial backers so he can enjoy the life of luxury he feels entitled to, yet is incapable of achieving through legitimate means.
The lies and obfuscation were off the scale, blatant and came with no apparent shame. It's all just a game to him and he just doesn't care what he destroys to get what he wants. A weak, yet dangerous man.