Excellent addition!
It would be great to have an option to invite a new Admin or site manager user to manage all sites rather than each one individually. This would also require an admin to see a list of all site managers to disable this access as well if needed.
Minor suggestion: There is a small loop on mobile where if you tap “site manager”, then “select site” it keeps the selector pop out open to until you tap to close the side bar list. It would be nice if you tap site manager it closes the list menu pop out to land you on the site manage page directly. I can send a screen recording if this isn’t clear.