The Court of Appeal isn't going to look fondly upon you if you try to get out of an arbitration clause with your lawyer because the engagement letter "had too many pages" so you didn't bother to read it.
That's true even if you had a previous engagement letter before your lawyer changed firms that didn't contain an arbitration clause. And even if you hire a legal ethics expert to say that the lawyer had a duty to specifically point out the new arbitration clause.
They like you to actually read these things. Arbitration clause enforced.