I agree that it's not cost-effective to re-invent the wheel. That said, you sound like a person who enjoys tinkering and learning. In which case, you will learn a lot by building an accounting system. You need to think about not just FileMaker, but how to structure things. Many people have spent lots of time thinking about this. See for example: https://martinfowler.com/eaaDev/Account.html
My advice is to start by building The Simplest Thing That Can Possibly Work, and iterate from there.
Also, you're making life more difficult by using such an old version of FileMaker. And it will make it more difficult for us to help you here. Not only do newer versions have useful new features, but security is much improved in newer versions of FileMaker. You claim to be concerned about security. So, save yourself some aggravation and upgrade!