If I did have fully pure Pu-241, would that be a better fuel than U-235?

Also not a physicist, but I assume from the fact that the OP is asking the LLM this question to trip it up, the point is that U-235 is better even if you have an abundance of both. It's scarcity of Pu-241 leads to the lack of data in training, not that it's actually better.

Again, really speaking out of my depth, but if there is a lack of plutonium training data, I would assume the answer would be the far more commonly described U-235. To respond otherwise means there is some existing association with Pu-241 being better.