To recap, I meant my example, where the maximum is at the even split, to refute the claim that any smooth utility function will obtain its maximum along one "most efficient" axis.

You only control a tiny portion of the money that gets donated to charity. If there's currently an equal amount of money donated to each charity, the ideal action would be to donate equally to each. If the difference between the amounts exceeds the amount you donated, which is more likely the case, you donate to the one that there's been less donated to. For example, if one charity has one million dollars in donations and the other has two million, and you donate a hundred thousand over your life, you should donate all of it to the charity that has a million.

There's no question of satiety of exhaustion at the level I currently invest in either.

I doubt that. You might still have fun doing each more, but not as much. If you chose to learn the piano before, but now choose to play tennis, something must have changed. If nothing changed, yet you make a different decision, you're acting irrationally.

Here's a closer "personal spending" analogy to charity:

Why is that analogy closer? It looks like it's in far mode instead of near mode, and the result is more controlled by what's pretty than what makes you happy. For example, if you got a $500 a month raise, you likely wouldn't save it all for the downpayment, even though there's no reason to treat it differently. If you got a $500 a month pay cut, you almost certainly wouldn't stop saving.