I think the only way that you can ever get a decent provision (in a school that is failing your child) is to prove they are failing your child.
and unfortunately, that does mean leaving your child to fail at school, while you first of all find something that does work, and then set about proving it works.
that something else might be private provision (OT, SALT, even tutors if the need is there) or it might be home edding (by which I mean working separately on the targets your child has, to show that they can be achieved at home, while not being achieved at school).
and then you wait, while your child does nothing at school - going along with it all, while documenting that targets are not reached at school, but met on X date at home (with proof), that attainment is stalled at school, but forging ahead at home - and that is hat you take to Tribunal.
not evidence that the school lied, or covered up, or overlooked needs. you need to prove that:
your child is able to learn
the school is not teaching them effectively
progression can be achieved by doing XYZ (preferably this will be on the same targets as have not worked at school)
the Tribunal is not interested in what has happened beforehand - they are not there to see that the school lied, or that the LA are failing children. they are there to ascertain what is the most efficient way to educate your child, and so they need to know how to go about doing that.
if you turn it into a "he said, she said" war, then the LA will almost certainly win.
if you (and it is unbelievable that it has to come to this) let your child fail, and prove that that failure is not the only way, then you have a better chance.