HI - Since the dashboard is not in the context of a course, removing the "student" or "teacher" view capability will work here. Instead, you need to make a Mask!
Basically: You'll make a new role that is applied at the block= level. It wont' have to have any permissions set (just 3 removed). Then place all of your teachers in it and on your Dashboard set up page / reset the dashboard, and then you're all set.
Create Mask
1. Site Admin - Users - Permissions - Define Roles
2. Click Add a New Role
3. Choose 'No Role' as the arch type
4. Give it a name (ie. Dashboard Mask)
5. Check off "Block" context
6. Leave everything else as "not set" and scroll down to the bottom of the page where you see the "Block" section:
Change the following three permissions to "Prohibit":
"Edit a Block's settings"
"View Block"
"Manage Blocks on a Page"
8. Click save changes.
Apply the Mask
1. Site Admin - Appearance - Default Dashboard page
2. Click "Blocks Editing On"
3. In the Course Overview Block, click the Cog and select "Assign Roles"
4. You'll see your Daskboard Mask listed and a '0' - click on "Dashboard Mask"
5. Add all of your staff to it (you will just have to maintain this as you get new staff members)
7. Navigate back to Site Admin - Appearance - Default Dashboard page
8. Click "Reset Dashboard for all users"
This works for me on Moodle 3.4