Custom dashboard widget – load style only for added widget

You could load the dashboard widget into its own document in an iframe if the problem is bootstrap css messing up other parts of the page.

This answer has example code for the iframe widget part of it:
Embed iframe or html page into dashboard widget

You just have to pass the iframe a url that doesn’t have the entire site in it. Something as simple as a page with shortcode that uses a completely blank template would be easy to do if you control the whole site. If it’s a plugin for general release, you might use a hardcoded page that loads the content with admin-ajax. More work, but cleaner, and could update on a timer, if that was a useful feature. I’d add some example code, but maybe I’ll wait to see if that solution works for you, first.