The get rich quick scheme I thought was well thought out, for the 'in universe' principles that had been laid out. One galleon converted to a lot of copper, so the mary sue could take gold from the muggle world, get it made into galleons in the wizard world, trade those for a metric shit ton of copper knuts, and then take those to the muggle world to be sold for a much larger sum of money than had been used to buy the gold.
As long as you don't expect it to work forever, it would be fine. The writing was terrible, but the character established all the nuts and bolts of the operation by 'just asking' questions to the diagetic narrator: pure gold was able to be made into galleons for a fee, banks would give you your money in knuts if you asked, and the prices would work for it.
The writing was jank and the protagonist narrator insufferable, but the conclusions he drew did make sense for the world he had been placed in, as appropriate for a 'rationalist' critique of harry potter.
Edit: the part where I just threw up was where the narrator had an immediate, perfectly-thought-out-but-the-writer-couldn't-come-up-with-an-actual-thing when mcgonagoll threatened to alter his memory, but he had thought of a perfect solution to that years ago. It reminded me of terrible ttrpg players who just ad hoc added parts to their backstory so they could be mary sues in a collaborative game.