Run each client in its own isolated organization and switch between them in a click. Their data never touches another client’s. And each client’s books work in their own currency (CAD, USD, euros or pounds), so a US-dollar company gets a US-dollar year-end.
Every client is a separate organization with its own security boundary. Nothing leaks between them.
A client with a company, a holding and personal books is still one client. Add as many entities as they need.
A client switcher in the header moves you between books instantly, with one monthly invoice for the whole firm.