"I'm happy for her to be autonomous to a point but don't want her to get to 18 with no qualifications and no options"
She won't. Provided she is happy and healthy, it is very very hard to imagine any scenario which leaves her without any qualifications and without any options in five years' time.
Without qualifications at 18, yes, it's entirely possible that that could happen. But how could she be without options? This idea that young people must achieve certain things before it is Too Late is an artefact of the school system. Elsewhere it hardly exists.
Let's look ahead to the morning of her 18th birthday. She has spent the previous five years relaxing, reading sometimes widely and sometimes deeply, and generally feeding her brain according to its particular hunger. She's done some voluntary work at the library and had a Saturday job at the corner shop, as well as helping around the house. She has some friends and some hobbies. She feels good about herself. She knows she is a capable person, and she knows she is loved. But she hasn't been near a proper textbook for years, she doesn't know how to write an essay, and she hasn't a clue about the content of the maths GCSE.
Now suppose this morning she realises that her life's ambition is to be a surgeon. Is she without options? Has she painted herself into a corner by frittering away her teenage years, abetted by her misguided feckless mum who thought it more important to let her recover her happiness than to "keep her options open" by getting a decent handful of exam results at a respectable age?
Of course not. Why would it be to late to do what she wants to do? It's no harder for an 18 year old to do IGCSEs from home than a 15 year old. In fact, it's a good deal easier if by waiting until 18 she has found her own motivation and focus.
One great thing about learning independently is that there's no such thing as too late or impossible. Kids don't have to contend with any nonsense about "we don't offer that to your year group", "we won't let you undertake this A level because your predicted grades aren't good enough", "the timetable doesn't allow that combination of subjects", "you can't join that course now because everyone else is too far ahead of you and you won't manage to catch up". And they don't feel daunted either, as they've seen older home educated kids do these things at unconventional ages. Sure, there are practical considerations and financial issues and plenty of hard work needed to achieve goals, but there are no insurmountable obstacles.
You've let go of some of the urgency already, in recognising that it may not be possible or desirable for your daughter to do the number of GCSEs that a school would have liked her to do, at the age which a school would think proper. 13 is young. Go off and do some surreptitious research if you like, surreptitious because at this moment your daughter needs complete freedom from contemplating exams even as a tiny dot on the horizon. Join home ed email lists and talk to other parents of teens to see what they are doing and how they are doing it. But try not to let her hear you gathering this information. She's only just escaped from all that.
I think you will find that as time goes by, your fears for your daughter's future will recede and recede until you actually feel comfortable with the idea of waiting as long as it takes for her to move forward in life.