It's likely that your padding amount is too low. 8k will need 64 pixels of padding outside each UV shell, so you need 128 pixels in the gutters between the shells. I would suggest adding more padding, just completely fill the empty gutters with padding. Photopea has a fast padding filter, and you may need to run it a…
painter won't do pixel perfect placement because you're projecting every stroke in 3d space, not editing the texture directly - if you want pixel perfect placement you use something that edits pixels directly (eg. photoshop) that's not the problem here at all though - if you tried to place antialiased text that size in…
If you have an actual pixel based map (it doesn't matter what kind or how big) applied to your object when you unwrap it, you can turn on pixel snap in the UVW Editor, the button in the lowest right corner (you might need to change it from grid to pixel). Then in the upper right change the drop down so it displays your…