This sounds really hard for both of you, and it's clear how much you care about her.
A few thoughts that might help. The fact that she's engaged with a therapist she actually clicks with is genuinely significant that relationship could be really valuable for unpacking some of the school pressure stuff too, not just general wellbeing. It might be worth gently asking the therapist (with DD's awareness) whether school stress is something they can work through together.
On the school communication her wanting privacy is completely understandable at 15, but there may be a middle ground. Rather than speaking to school about her struggles which can feel exposing, some parents find it useful to ask school quietly about what accommodations exist for students with DLD without it necessarily being about their specific child first. That way you're gathering information rather than flagging her.
On the GCSEs themselves fewer GCSEs is genuinely a valid path and doesn't close as many doors as people think. Most colleges and employers care about English, Maths and a handful of others. If reducing the load takes the pressure down enough that she actually passes the ones she does sit, that's a much better outcome than struggling through all of them.
When she is ready to revise, Kingsbridge Education and Save My Exams are both really accessible resources good quality, not overwhelming, and she can work through them at her own pace without it feeling like school. Sometimes having control over how and when she revises makes a real difference for young people who find the school environment stressful.
But really you're doing the right things. Therapy, listening to her wishes, taking it seriously. That matters more than any resource