A more easier approach would be to have an integration with an external database like Firebase. 
Privacy rules do go a far way handling security. You can also offer sub apps for each instance on a professional plan if your clients want more assurance of data seperation.
P.S. only if user is logged in can NOT be a good privacy rule. You will need a reference point, for example Data Type Company.
If Current User’s Company is This User’s Company - then allow read/write access.
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