Has "draw cards if an opponent has more cards in hand than you" been considered fot white? Keeper of the Accord does the same for lands.

It would be too efficient for white. White already has a weenie strategy that lets it easily dump its hand.

If white is proactive and not reactive, what's the design philosophy behind cards like Keeper of the Accord?

I think you’re confusing what I meant by “reactive”. I cast a spell in reaction to an action my opponent has taken. White can generate white effects based on a board state.

It seems that Commander Legends isn't doing that much different for white. Monarch is pretty cool in white, but I expected more of a shift in its color pie. Keeper of the Accord seems to be the only big card. Is it being saved for last or is this it?

The color pie adapts to the environment, but it takes time and adapts slowly. A lot of time it’s us dipping our toe trying something out and then expanding on things that work. In Standard this happens faster because things rotate out and the environment is smaller. Shifting an environment with 27 years worth of cards in which nothing leaves the environment is not something that can happen quickly.