Support isn’t automatically reduced post 16. Less provision doesn’t automatically mean encouraging independence. That is a myth LAs like to perpetuate in order to save money.
In EHCPs, the provision detailed, specified and quantified in F must be provided. The LA is ultimately responsible. That still applies post 16.
You don’t need legal advice at this stage (and many don’t at all).
In the autumn term of Y11, there should be a phase transfer review. Then the phase transfer EHCP finalised by 31st March of Y11.
In preparation for the phase transfer review, as DD moves into and through Y10 and right at the beginning of Y11, it is a good idea to ensure you start to look at placements and ensure evidence is all in line.
You don’t need to fund independent assessments at this stage (and, again, some don’t at all).
Right now, the best thing you can do is ensure preparation for adulthood is discussed as part of the Y9 AR. You might find this booklet helpful when looking at PfA. I will also post some screenshots from another document - I can’t upload it because the link isn’t available on the internet anymore and MN doesn’t allow links to documents.