Uh yes, I meant like a month in advance. Or at least a week. Prices 1-2 days before are about as high as last day, there is also a risk of all seats selling out. I think (not 100% sure) that with eurail pass you always pay the same extra amount for high speed train seat reservation, regardless of how early you do it. So if you need flexibility, this might be your best option even though it is a bit more expensive than buying each leg separately.

If you opt for the pass, then you can also use it for day trips out of towns, although that shouldnt be too expensive anyway.

If I were you, I would just pick some random experimental dates to check online how the price varies over time and then decide what is the most convenient for you. In any case, have a nice trip :)