I think the best approach would be to propose the minimal interface, and then multiple levels of convenience on top of that, and see what LEWG is comfortable with.

Level 0 would be the function I've shown, just taking the integer and fraction digits, and the pre-parsed exponent.
Level 1 would additionally take the exponent digits and sign, in case the user doesn't want to preparse the exponent.
Level 2 would additionally take a character which is ignored anywhere, to enable digit separators

I'm not sure where leading sign characters would come in, or if they're worth supporting at all. The user can just as easily call std::copysign afterwards as passing the sign into std::parse_float, so it appears to be of no value.

There are a few adjacent problems, like making the thing constexpr, but that should be done in sync with making std::from_chars constexpr, and it's a separate issue. Supporting char8_t, char16_t, and char32_t are also adjacent issues and should be done in sync with std::from_chars.

Anyhow, if you'd like to co-author, I'd be happy with that. I'll send some more details by the end of the week.