The fact is… Discourse is not built with Bootstrap. So, what you want to achieve will involve a lot of friction.
There are over 400 buttons used in the Discourse UI. If you were to change their default classes, you would end up having to override almost 200 templates and that’s just for handlebars. That is going to be a maintenance nightmare.
A more reasonable approach is to reset the button styles you’re not going to use and adjust the rest based on Bootstrap button styles.
The base styles for all the buttons in Discourse can be found here
discourse/buttons.scss at master · discourse/discourse · GitHub