Resolution and print enthusiast checking in.
If it's on tapas, remember that tapas has a limit of 960px wide, which makes everything look bad. The file there is also a screenshot, which naturally compresses images down no matter what the dpi is. The thing here is them more you compress (your 600dpi 10kx10k image down to 72dpi 960x960ox image for example) the more likely it will look like garbage. The computer has to compress too much, so you run into issues with 'jpeggy' looking artifacting like you tried to resize an image too much.
That, and if you crank the res TOO high your stuff can end up looking muddy and blurry anyway. I know, ironic. Don't work higher than 350dpi for colour unless your machine can handle it, but there's no good reason to do it for print or for web export.
Try working in CSP vector layers for lineart, they tend to look less blurry or jagged than regular lines. You can also turn anti aliasing off, which might give you the sharpness you crave in 600 dpi - this may be the hot tip you're looking for. However I wouldn't say that'd serve your simple lineart style particularly well, and it'd be a whole lot of effort for maybe not much payoff.
