you can try more top layers.

Or you can try solid printing - ie: no infill pattern just solid plastic.

Fonts do matter, some that look really good on paper can have really thin sections that the slicers just don't seem able to handle that well.
10mm square is a pretty large letter - so thin sections should not be a major issue.

But anytime you have gaps in your top layers like that - then more top layers and denser infill is usually the answer.

Keep meaning to look at the latest prusa slicer.