There may be a third option which would be to create a “window” fund only for the repair of the windows belonging to the units that have opted out for now, to be spent when they need it.
But really, there are very few options that make more financial sense than persauding the backsliders just to get it done.
A spreadhseet that showed how much it would costs to get four sets of windows done at once, including scaffolding etc etc, compared to doing it all twice, and adding in a figure for inflation for the second round, might be persuasive.