My team is facing a similar situation - thanks for chiming in here!
The dummy account workaround is really the only realistic option here - giving out an existing admin’s API key will give anyone with that key access to any private boards that admin has.
The biggest issue (beyond the API key only being accessible to admins) is the overall admin structure within Monday. It should be a pretty basic feature - delegating different levels of admin access (ie. super admin has access to everything, all other admin features are delegated on a case-by-case basis). As it currently works, there is only 1 level of admin access - full access - which in reality enables any admin to remove board owners and place themselves as the owner.
Monday should make this a priority - break the admin features into several groupings (billings, team membership, API access/security, branding/customization) and allow the master admin to delegate select admin capabilities to certain users based on their needs.
Hope this can be put in the works relatively soon - the admin feature is too powerful as is and creates situations where teams need to make “dummy” accounts just to access certain features.