Bootstrap best practices?

I do both web design and development. I usually prefer to write my own CSS and not use any frameworks. On my current project the client said after I finished the design that it would help their dev team if I could also code the HTML & CSS. Their requirement was to use Bootstrap 4.

I said OK and got to work. However I have some questions about best practices when working with Bootstrap. For example, say I want to have a .navbar-text that needs a left border with a certain non-theme color. My questions are:

• Should I create a new class for this, or just overwrite the .navbar-text class? Usually I follow BEM and would create a new class for this, but I'm not sure if that wouldn't be confusing in this instance.
• Since I'm already adding a CSS rule for the border, should I continue to add Bootstrap utility classes on the element, or should I just add CSS to my new rule? Say I want to add some left padding, should I add the pl-2 class to the element, or should I add padding-left: 0.5rem to my CSS rule?

I want to make my code as easy as possible for their dev team to work with and I'm not sure how to proceed. Any advice would be appreciated.