"and its intended to serve small businesses in accounting"
Are you trying to therefore create a General Ledger , that will enable small businesses to do their accounting online.
First step is to check how existing accounting softwares are 'modelled'. There are industry standards for UML / Entity Relationship Modelling which are implemented from the ground up by the Enterprise Companies like Oracle and SAP.
Research the Data Model for Oracle Financials , which should be based on the Industry Standard.
I am creating a Retail Tech solution from the ground up, and i am using the published ARTS data model from OMG as the data model,which is the industry standard for retail tech solutions including Oracle.
Of course its a long road ahead, as the data model is ginormous,but i start by focussing on the core concepts in ER modelling, working up to the actual database schema.
am thinking of implementing a single database that will track users and their linked database
This is a nuance / feature of your solution and you can plan better in a bottom up manner, rather than the top down way that you are using to design your accounting solution.
The point from an enterprise perspective is that its the Business Need that drives the innovation , the technology, and the implementation, not the other way around.
To offer an on-demand Accounting Solution online to multiple customers, you have to check how existing solutions like SalesForce is built, and once you have a general understanding of the same, you can think of improvements / modifications.