You may find https://markusvoelter.medium.com/the-evolution-of-decision-t... helpful. He also has a lot on DSLs which is worth reading.

There is also a more powerful version of Decision Tables called Tabular Expressions (from D.L.Parnas) which you can use.

A nice succinct reference to Formal Methods is Concise Guide to Formal Methods by Gerard O'Regan.

I think simplified versions of Formal Methods can be used profitably for your use-cases. Start with Assertions, Design-by-Contract and Decision Tables.