A feature freeze that lets you tackle your mountain of tech debt at once will never happen. Don't count on it.
Instead, chip away at tech debt constantly. When you're working on a feature in the area, make that component better. When you're using a dependency, update it. See some unclear logic? Rewrite it.
This concept is also referred to as 'gardening', the 'scout rule', or the 'campsite rule' - leave things better than you found them.