If you've already decided you're going to pay for the seats, then why are you waiting?

If you pay now, you get your pick of what's left.

If you wait until there's only one window seat left, then you may be paying for the worst window seat in the cabin.

For that matter, if you see two left and you decide to wait until there's only one left, you may find that both of them go at once because a family of four have booked and taken both remaining window-aisle pairs at once. Then there will be no window seats left.

It seems that this really really really matters to you. If so, why are you trying to game it for the sake of trying to save a few quid?