Or would 3 academic subjects plus extra-curricular drama activity be better?
This really is a "How long is a piece of string?" question, and to be honest, I think you're allowing the tail to wag the dog.
If your DD wants to be an actress, then she needs to be involved in youth theatre of some sort, or amdram, or whatever. But stuff like this should be done because she wants to do it, because it's a normal part of childhood/teen years life enrichment, because it is what she wants to do - all the reasons for doing hobbies. Not to get points or qualifications.
This is the problem with the exam culture - we forget what exams are there for. We have lazily got into the habit of thinking of achievement at exams as an end in themselves, rather than a simple test of how much has been learnt & how well, at any particular point in the learning.
LAMDA grades will make no difference at all to a conservatoire audition. Talent will. The work she's done for the exams may help, by helping to articulate & develop her talent. OTOH, I find students who've done that grade/exam work can be a bit elocution-y & not really flexible in creativity.
Most university Drama departments NOT conservatoires (drama schools don't necessarily require Drama or Theatre at A Level because of the appalling mess that the eBacc is making of creative arts education. But if there's an opportunity to student Drama at GCSE and then A Level, and your DD wants to do it then she should go for it.
If she wants to do the LAMDA stuff, she should do it. But for her own self-cultivation & development NOT with an eye to post-secondary education. It really won't make much of a difference there.