Those glow and burn effects are really cool! Thank you so much for this!
One thing I've noticed is that, in fonts where characters might overlap, there's a black vertical line in the shader effect:
Would there be a way to fix this?
So yes, you can change the variables of the shaders using the method included in the Ren'Py official documentation. I need to take the time to write up a cheat sheet for the shaders so people can see the variables without needing to dive into rows of code.
https://www.renpy.org/doc/html/textshaders.html#specifying-text-shaders
Most of everything is customizable, let me know if there's anything you can't easily change and I'll see if I can make an update to expose it or make some more easily utilized knobs.