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.
You're correct. A 0.2mm nozzle would certainly improve things. But it would also make printing those tabs unbearably long.
I wish my company had bought a 5-head Prusa XL: then I could have loaded black PLA and white PLA in two heads with 0.2mm nozzles, and a separate feed of black PLA in a third head with a 0.4mm nozzle for the rest of the parts that don't need to look nice. But... ours only has two heads and it's 0.4mm on both, because all the other parts we prints just don't need finer details.
Try reducing the line width especially for the initial layer. I googled about Arachne line generator and it looks like it's the default wall generator in PrusaSlicer as of v2.5. I can't tell if you're using a textured build plate or not but you could also try swapping to a smooth one (or vice versa) and seeing if that makes any difference. You don't even need to print the whole tab for testing, just the first few layers to see if you're on the right track.
You could also try using a 0.2mm nozzle just for the white filament and seeing if that helps.