There are so many variants.
Go on somewhere like the DIBB or DIS forums. They will give you tips and ideas and will also be able t9 help with budgets.
That said, there is no doubt that Disney has become more expensive in the last 8-10 years and not just against inflation. They have added extra costs that used to be free and taken away offers that, imo, helped British families with the cost.
I think these additional costs and removed offers does make Villa/car hire or off site hotel/shuttle/Uber/Lyft more attractive as less to lose (previously on site would get your bigger windows to book free fast passes for rides and often gave you free dining plans but now there is no such thing as a free fast pass and the dining offers aren’t as comprehensive).
Staying off site will significantly reduce your food bill too. But be aware of the eye watering high car parking fees at the parks.
If you are entirely stuck to English school holidays, avoid Easter and Christmas. Look at the last two weeks of August when American schools are generally back . If your term starts first week of September, try and push the return into that too as flights will be cheaper.
From age 10 you child will be a Disney adult (tickets and food).
Look at flying into Melbourne rather than Orlando. Or consider indirect flights.
Unless huge Harry Potter fans or love a fast roller coaster/ride, don’t bother with Universal (although I think Universal is much better value but obviously won’t give you the Disney feel that your daughter seems to be looking for). Day tickets are so expensive for both Universal and Disney so tickets for 5/10/14 days are usually better value.)
If only doing a few days in the parks avoid the weekends, look out for American holidays and plan your day around opening times, show times and work out whether Lightening Lanes (previously free fast passes) or are worth paying for. And look at Busy Day Guides. All these things help you budget to make sure you get the most for your money. (Doesn’t make it cheaper, I know, but help you work out if X is worth the cost.)
Consider a week doing Disney and week on the coast (if doing last two weeks in Aug, maybe do the coast first as I think by the last week of Aug/into Sep, you will go into the realms of mid season rather than high season thus reducing the cost.
But still taking all the above into account, I think it is an expensive holiday. Only you can answer if worth saving for a couple of years and forgoing other holidays to do it.