Indeed multitenancy can be done without this module but it could help you understand multitenancy.
You first mistake is that an employee has an account not is an account. Make a one on one reference between account and Medewerker(employee) and do not inherit.
Now your Klant (client) object is your tenant object. This means that every entity you create needs a reference to that tenant object. Use after create actions to always attach the right Klant to each and every object.
You can then use this reference in your access rules where you check that a user can only read entities that have a reference to the same Klant(Client) as that user.
Regards,
Ronald