o.p. Just curious: how long have you been skating? If you were much, much smaller and lighter when you started, maybe the Mystiques were OK while you were at the "march and glide" phase...
I suspect you have prior athletic experience, in something else that requires good balance - e.g., performance dance or gymnastics?
Ic3Rabbit: according to a brief web search, there are multiple skate stores around Sydney. Maybe you could suggest one or more specific stores?
Do you think she should tell her fitter if she learned all that in a year or two, so the fitter doesn't underboot her? (On the assumption that fairly rapid progress might continue, even if she only skates 2 or 3 hours a week.) There is a lot of difference in price between the two boot models you have recommended, and maybe the fitter would take the rate of progress into account.
o.p.: I hope you can find transportation to the appropriate store in Sydney. I know that sounds like a pretty long drive, but it really isn't, compared to how long a lot of skaters have to travel to find a good store. Make sure you make an appointment, if they can give you one, at a time that lets you try skating in the new boots and blades, if they have them in stock. If they have to be ordered, a lot of people feel it is best to get the good store to make the final adjustments too. Some adjustments are virtually impossible to get exactly right without testing on the ice. And many people feel that blades often arrive shaped a little wrong, and you need a first class skate tech to reshape them right. Also, sometimes boots and/or blades are defective. If they need to be returned, the original store is the one best able to take care of that, and maybe will even do it at no extra charge.
It's unfortunate that between the cost of travel and the cost of the boots and blades specified, they may not be quite as "affordable" as you hoped. But if you have been paying for private lessons, and you can only have as little practice time per week as you say, you may make more rapid progress than would be possible with the Mystiques, and are therefore making more efficient use of those lessons and practice time.
Especially if the new boots help you avoid an injury. One way to interpret the pain you have been feeling might be that your body is telling you that it doesn't feel safe with the current boots. That's not the only possible source of pain, but sometimes pain occurs if you put more stress on the body than it is able to take. (I hope it doesn't mean you have already created an injury.) Or at least that is my personal experience - though I'm not as athletic as you are.