How do I implement bootstrap's panel component for a specific block / block region in my bootstrap (sub) theme?
Quick-and-dirty way: Uncheck all the boxes to display default classes in Views, and then use a twig template specifically for that views block to write the CSS exactly as you want.
You can find the Views twig templates in /core/modules/views/templates.
It depends on how you configured the view, but if it's an unformatted block, you want the unformatted template, views-view-unformatted.html.twig.  Copy that file from the Views template directory into your subtheme's directory.
Next, you have to rename the file so that it only applies to the block. In Drupal 7, Views was nice and gave you the suggested theme hooks, but in Drupal 8, you have to figure them out yourself.
The rule is TEMPLATE_NAME--VIEW_MACHINE_NAME--DISPLAY_MACHINE_NAME.
If you go to edit the view in the admin UI, you can get this info from the URL.
/admin/structure/views/view/VIEW_MACHINE_NAME/edit/DISPLAY_MACHINE_NAME.
Once you rename the file, you will need to rebuild the cache to get Drupal to use it.
Sometimes you may need to customize other views templates as well (a single view can use multiple templates); you can do that in the same way as described above..