Thank you for the input. After looking at the code again, I think you are right, most strings used are easily converted to expressions as they more or less already are, the few that are not valid R Syntax are already parsed in some way so it should not be an issue to adapt that for backwards compatibility.
There are several advantages least of all is following a more R-like API. The main advantage would be easier code, better maintainability, and future-proofing of the backend.
On the users' side, it would be a more natural workflow (the package is simstudy, we use formulas for simulating data, inputting them as an expression is much cleaner imho) and allows for easier implementation of custom distributions and more flexibility.