As you found, for monitoring, you can remove the sysadmin role, but have to replace them with quite a few. Check it out here: Success Center
Keep in mind that *some* functionality may be missing should the monitoring user not have sysadmin.
For the repo, sysadmin is used to create the new database and schema owner. After that, sysadmin for the repo is not persisted or used.
Are you in a position where you absolutely don't have access to sysadmin credentials?
In addition to Rob's information, there is an option to register a DB without a sysadmin user via the mass registration interface and by leveraging a domain account.
Check out the Success Center article Register a SQL Server instance for DPA monitoring without sysadmin credentials