So I played with all kinds of settings in PrusaSlicer. Nothing changed anything.
The only things that did improve the outcome some was:
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Forcing the letters to be printed first: then the letters are smooshed and bleed into the background instead of the other way round, which arguably looks better / more legible. Nothing to write home about though.
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Dropping the first layer's height to 0.1mm (the other layers are 0.2mm high): that improves the letters a bit.
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Dropping the first layer's height to 0.05mm: because the first layer is so thin, it becomes kind of translucent and the wider white letter beneath it sort of show through. The net result is that it drops a kind of gaussian blur onto the lettering, which actually improves them - especially at a distance.
Other than that, there's just nothing for it. And half of the suggestions I got concern other slicers, and I couldn't find them or equivalents in PrusaSlicer. Oh well...
I guess that's as good as it's gonna get.
I've dealt with this exact issue. Increasing font size (increasing smallest feature size) helps. As does printing the letters first, as you have found. Although sometimes the letters dislodge and become floating letters a la Alice in wonderland. Depends on your bed adhesion.
Then linewidth on the perimeter touching the letter is something you could look into. (Reducing linewidth to reduce overlap/bleed)
It won't be one magic solution, rather a combination of several factors all contributing to a more acceptable result.