In regards to restricting access to the /web/database/selector and /web/database/manager pages there are a few options. You could try the following module that redirects from /web/database/manager to the website login for those without access rights to see the /web/database/manager page:
or you could set up Nginx as a reverse proxy in front of Odoo and simply use Nginx rules to restrict access to those pages to certain fixed IP addresses by using the example i've provided in the following forum post:
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