Maybe the "Multi-DB Synchronization" from "Serpent" is helpful?

https://apps.odoo.com/apps/modules/12.0/base_synchro/

The question has many answers, here is my first impression of a suitable solution:

Your client may use one database with several clients setup as "Child" companies.  This provides your client with a single set of  accounting reports at year-end while allowing each vertical to be fully independent otherwise.

Each Vertical may simply ignore any modules they do not require.  If you are customizing, then simply do not customize any original code; always create a unique module that may inherit from the original.

For custom modules, my first thought is a top-level menu item for each vertical.  Users of each vertical will belong to a group, that group will have permission to SEE only the menus that they are authorized to use.  So "Restaurant" menu is visible only to "group_restaurant" staff.

Each module will have similar permissions and so each user will see only the modules and menus that their group allows.