Two things that I’ve noticed and that have helped me in similar but not such extreme cases.
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JOSM is a massive time saver once you learn how to use it. It’s made editing a lot quicker when I’m working on similar tasks.
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Try break it down into small tasks for yourself. It’s a lot easier to work on smaller pieces of a city/area than trying to cover everything all at once.
Ireland is currently working on something similar (example), I’m not sure how easily you can create your own private tasks for your country/area of mapping.