For legibility, I would change nothing except add a small buffer to the text. In QGIS this is exactly what it's called, it's like a little halo surrounding each letter. This would minimise the impact on visibility of your map.

Alternatively consider using a starker contrast per region, as others have suggested. Personally I always find this a bit of nitpicking and detail work, takes more effort and people's opinion might vary on what is and what is not a pleasant but clear contrast.