The first time someone did __pyproject__ = f"""...""" you’d regret that leniency 
Honestly, a lot of this is covered in PEP 722’s rejected alternative “Why not use (possibly restricted) Python syntax?” I’m not trying to say that PEP 723 is doomed because I was right all along, but I do think you should start by making sure you’ve got good arguments as to why the issues described in that section don’t apply to PEP 723. There’s a certain amount of subjectiveness to what I put in PEP 722, certainly, but I tried to be objective enough that a simple “I don’t agree” is likely to be missing something.