FI think using multiple organizaitons would work perfectly fine for your use case.
Fhowever i think the decision is dependnet on your needs in regards of security. if you use different instances, you have a different data pot for the businesses, they will not know something about each other, but you will also loose the possiblity to use a user from organization and give them permission in another where they do not belong.
Fif you use all organizations in one instance, you have to possiblity to also use one user in an organizatino and give it permission across multple. also you only have to manage the service once in your own orgnaization and you can grant it to your customers