Oh man, how didn't I find this out before?
So an hour back, I was fooling around on my choice of a free art program (MediBang Paint Pro). I just wanted to explore the options, because on mobile, MediBang has brush settings for opacity by pressure and stuff. I didn't know if computer MediBang had them so. And then, I just cjamced upon the "create new brush" button.
Turns out, YOU CAN MAKE BRUSHES! They all come with all sort of cool options, too! And yes, opacity is available.
Needless to say, I will be playing with it some more now OWO