It's okay, don't rush it, I found a solution for me (mentioned in the edit above, setting color to gl_FragColor). Thanks for your response.
Here is the sprite link, though, as it turned out it doesn't matter really.
I've made a sample project to show it off link. I did it here with a standart sprite from "The Question", which is 500x700, so resolution isn't the problem, I guess.
And sadly no, setting v_tex_coord to a_tex_coord doesn't help. Really, it seems like a problem with renpy? I will open an issue on github some days later if we will not figure out the cause.
I also thought that it maybe show like that only when redacting the game project, but no, it is the same when the game is build, so it is either a problem with the shader (somehow?), renpy or my gpu (probably not, but would be nice to know if it's the same on your pc, there's always a chance, it is AMD by the way).
Update: just asked my friend has an Nvidia 1060 and it looks okay on his pc, I have an RX 6600 and it looks blurry...
Update2: opened an issue on Github link