RE: Hide Dashboard & Diagram Button
The 'Open Dashboard' button opens up an entity specific dashboard. These entity specific dashboards come out-of-the-box with some of the standard entities, e.g. Accounts, Contacts, Leads, Quotes, Invoice and Opportunity. You can read more about these dashboards here: https://www.inogic.com/blog/2018/05/new-entity-specific-dashboard-in-dynamics-365-v9-0/
One way to get rid of the 'Open Dashboard' button is to delete the dashboard for that entity, but obviously you should think about this carefully before you delete anything and take a backup of the dashboard in another solution before you delete.
I tried removing all the security roles assigned to the dashboard, but there must be at least one dashboard which is enabled for fallback, so you can't uncheck that box if there are no other dashboards enabled for fallback.
I tried this and it worked successfully for me and the 'Open Dashboard' button is gone.
To find the entity specific dashboard, open the default solution, find the entity you want and expand it, click on Dashboards and the dashboard will be there (see below).

Regarding the 'Show Charts' button, you could try the same thing, by deleting all System Charts for that specific entity but always remember to take a backup first and discuss with your client if it's that big of an issue if the button is still there.