Can you tell me the name of the fonts you're using there and the version of Ren'Py you're on? I went through and I couldn't reproduce it in 8.3.4 with any of the shaders in the pack, even with the more scripty ones.
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.