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.
This is a resolution issue, so think about X & Y settings rather than Z settings like layer height. You need to reduce the line width and/or use a smaller nozzle like 0.2mm. You might also have a bit too much squish on your first layer which you could measure by printing a line and measuring it with calipers and comparing the slicer value to the actual value to see if there's any difference.
I'm not familiar with PrusaSlicer but Bambu Studio has a wall generating setting called "Arachne" which does a better job of generating the small features/walls you need for text even with a 0.4mm nozzle. You might see if there's a comparable setting for Prusa.
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.
Longer than its been taking to solve this issue without it? Just bite the bullet and get the results you want.
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.