What is the goal here? Are you just trying to simplify ast.c?

My concern is that there are many, many edge cases, and that you'll be unknowingly changing the behavior of f-strings.

One of the goals of the f-string specification is for a simple third-party parser to be able to lexically recognize f-strings just like normal, raw, or byte strings. It should require no change to such a lexer except for adding "f" where "b", "r", or "u" is currently allowed. I do not want to break that design principle. There are plenty of examples in the wild where this design was leveraged.