As I recall, Fleur's speech had only a few characteristic misspellings which people tend to identify as a french accent (dropped 'h', changed 'th', etc) instead of being actually phonetic. There were some misspellings, but they didn't get in the way of understanding what she was saying.

When the misspellings turn the sentence into a cipher, you know you've gone too far with the accent... :-)

My books are still in boxes from moving, but I'll try to dig out one I have on dialogue to post its example of a phonetically written southern accent, and a "grammar and just the flavour" of the same sentence. The phonetically written one is incomprehensible unless you already know what she's talking about.

One problem with I haven't seen writing books address is, one of the problems with writing phonetically (and without using IPA) is: what if you and your reader have significantly different accents? They'll read something written "phonetically" in a different way than you intended it. What does that 'a' sound like?

It's like an American writing a Canadian accent, and using "aboot". To a Canadian reading it, it just sounds silly - we don't say aboot - 'oo' and 'ou' are entirely different sounds to us as well, even though to an American ear it sounds like we're saying something closer to 'aboot' than how they pronounce 'about'.
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Who needs to be big and burly when you can just apply physics?