@mothersdaywoe you are in a really difficult situation, it is so hard with DC who are struggling with their mental health. I have a graduate DS who is neurodiverse and I've spent a significant part of the past four years trying to support him with his mental health and it's been the hardest thing I've ever had to do as a parent. No-one ever lies in the post natal ward wondering if they're going to have to deal with self-harm and suicidal ideation. You just want them to be happy and thriving and it's awful when you know they are neither.
I get that you want your DD to feel like she's progressing career wise but posts like the one above do make a lot of sense. With my DS, I'm well aware that a graduate job will be a distant prospect - like your DD he's highly able and has a good degree but he's a long way from being able to compete in any kind of graduate jobs market, let alone given the current bunfight.
But he is working a minimum wage type hospitality job in the meantime and it's doing him so much good. It's a reason to get out of bed and clean his teeth, at least. He's earning money, gaining useful experience, spending time around people his own age. He's even got a bit of a social life out of it. Does he want to do bar work forever? No of course not. Will future employers care about 'transferable skills'? Maybe not. Will it lead to graduate opportunities? Nope. Is the job helping his mental health and general feeling of self worth and positivity right now, today, on an ongoing basis? Yes absolutely!
FWIW, and to go back to your original query, yes I'd absolutely pay £500 to someone decent to help with CV development, interview skills, confidence building, networking and so on. In time, I may well do the same for DS. But it sounds like your DD has a long way to go before she'd be able to access that kind of thing.
I wish you and DD all the best.