I think you've put a lot of work into the RP scale. It seems to me that that would form a good upper limit for what people might use to describe their characters, but I'm not sure that amount of detail is necessary. Plus, I wouldn't say I prefer a specific tone; it depends on what fits the character concept. Even with the light/medium/heavy scale, it's a spectrum, people interpret it differently, and SGs might cater to a range of styles. I do think that choice of medium is a good thing to mention: in-game is more immersive, discord is more mobile-friendly, and the forums make it easier to look up old posts.
I like a mix of long form and one-offs. give examples. server wide arcs seem like they'd be unwieldy but I've never been involved in one. Of the RPSGs I've been in, they limit arcs to the SG itself and coalition partners.
TLDR: I'm not sure that much granularity is necessary.
I think what I'd want to see in RP personals ads is what tone(s) the person is looking for, what character concepts they have, RP format, and timezone.