Plenty of accounting software that can do multi-tenancy and multi-entity exists that can be self-hosted.

Accounting software is not novel, multi-tenancy and multi-entity is not a novel feature.

There are plenty of accounting platforms that do a lot of this, if not most, and are self hosted or you can run in your own cloud.

Odoo is an easy one that comes to mind, plenty of others.

GNUCash isn't really a business accounting platform.