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.
Just a thought: Try making the first layer of letters empty so the letter fill is actually a layer 2 bridge.
A trick I have done is printing the first layer solid in a transparent filament, then layer 2 as 2 color.
I recommended this on the first post, surprised they didn't try it.